Method for transmitting synchronisation reference signal for device-to-device (d2d) communication in wireless communication system and apparatus therefor

ABSTRACT

The present invention relates to a method for receiving a synchronisation reference signal for device-to-device (D2D) communication by a first terminal in a wireless communication system and an apparatus therefor. More specifically, the present invention comprises a step of receiving a plurality of synchronisation reference signals including a first synchronisation reference signal and a second synchronisation reference signal over a D2D synchronisation reference signal transmission cycle, wherein the first synchronisation reference signal is transmitted by a cluster head for D2D communication and the second synchronisation reference signal is transmitted by a second terminal that belongs to a cluster for the D2D communication.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method for transmitting a synchronizationreference signal for device-to-device (D2D) communication in a wirelesscommunication system and an apparatus therefor.

BACKGROUND ART

A 3rd generation partnership project long term evolution (3GPP LTE)(hereinafter, referred to as ‘LTE’) communication system which is anexample of a wireless communication system to which the presentinvention can be applied will be described in brief.

FIG. 1 is a diagram illustrating a network structure of an EvolvedUniversal Mobile Telecommunications System (E-UMTS) which is an exampleof a wireless communication system. The E-UMTS is an evolved version ofthe conventional UMTS, and its basic standardization is in progressunder the 3rd Generation Partnership Project (3GPP). The E-UMTS may bereferred to as a Long Term Evolution (LTE) system. Details of thetechnical specifications of the UMTS and E-UMTS may be understood withreference to Release 7 and Release 8 of “3rd Generation PartnershipProject; Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), basestations (eNode B; eNB), and an Access Gateway (AG) which is located atan end of a network (E-UTRAN) and connected to an external network. Thebase stations may simultaneously transmit multiple data streams for abroadcast service, a multicast service and/or a unicast service.

One or more cells exist for one base station. One cell is set to one ofbandwidths of 1.44, 3, 5, 10, 15 and 20 MHz to provide a downlink oruplink transport service to several user equipments. Different cells maybe set to provide different bandwidths. Also, one base station controlsdata transmission and reception for a plurality of user equipments. Thebase station transmits downlink (DL) scheduling information of downlinkdata to the corresponding user equipment to notify the correspondinguser equipment of time and frequency domains to which data will betransmitted and information related to encoding, data size, and hybridautomatic repeat and request (HARQ). Also, the base station transmitsuplink (UL) scheduling information of uplink data to the correspondinguser equipment to notify the corresponding user equipment of time andfrequency domains that can be used by the corresponding user equipment,and information related to encoding, data size, and HARQ. An interfacefor transmitting user traffic or control traffic may be used between thebase stations. A Core Network (CN) may include the AG and a network nodeor the like for user registration of the user equipment. The AG managesmobility of the user equipment on a Tracking Area (TA) basis, whereinone TA includes a plurality of cells.

Although the wireless communication technology developed based on WCDMAhas been evolved into LTE, request and expectation of users andproviders have continued to increase. Also, since another wirelessaccess technology is being continuously developed, new evolution of thewireless communication technology will be required for competitivenessin the future. In this respect, reduction of cost per bit, increase ofavailable service, use of adaptable frequency band, simple structure andopen type interface, proper power consumption of the user equipment,etc. are required.

In order to assist an eNB and efficiently managing a wirelesscommunication system, a UE periodically and/or aperiodically reportsstate information about a current channel to the eNB. The reportedchannel state information may include results calculated inconsideration of various situations, and accordingly a more efficientreporting method is needed.

DISCLOSURE Technical Problem

The present invention based on the above-described discussion provides amethod for transmitting a synchronization reference signal for D2Dcommunication in a wireless communication system and an apparatustherefor.

The technical objects that can be achieved through the present inventionare not limited to what has been particularly described hereinabove andother technical objects not described herein will be more clearlyunderstood by persons skilled in the art from the following detaileddescription.

Technical Solution

In an aspect of the present invention for solving the above-describedproblem, provided herein is a method for receiving a synchronizationreference signal for device-to-device (D2D) communication by a firstuser equipment (UE) in a wireless communication system, includingreceiving a plurality of synchronization reference signals including afirst synchronization reference signal and a second of synchronizationreference signal in a D2D synchronization reference signal transmissionperiod, wherein the first synchronization reference signal istransmitted by a cluster head for D2D communication and the secondsynchronization reference signal is transmitted by a second UE belongingto a cluster for D2D communication.

A transmission period of the second synchronization reference signal maybe configured to be different from a transmission period of the firstsynchronization reference signal.

A transmission resource index of the second synchronization referencesignal may be configured to be different from a transmission resourceindex of the first synchronization reference signal.

The first synchronization reference signal may be repeatedly transmittedto be equalized with a boundary of a subframe when a random backoff endtime of the cluster head is not equal to the boundary of the subframe.

Resource allocation information for the first synchronization referencesignal and the second synchronization reference signal may betransmitted over a physical device-to-device synchronization channel(PD2DSCH).

In another aspect of the present invention for solving theabove-described problem, provided herein is a first user equipment (UE)for receiving a synchronization reference signal for device-to-device(D2D) communication in a wireless communication system, including aradio frequency unit; and a processor and wherein the processor isconfigured to receive a plurality of synchronization reference signalsincluding a first synchronization reference signal and a second ofsynchronization reference signal in a D2D synchronization referencesignal transmission period, and wherein the first synchronizationreference signal is transmitted by a cluster head for D2D communicationand the second synchronization reference signal is transmitted by asecond UE belonging to a cluster for D2D communication.

Advantageous Effects

According to embodiments of the present invention, a synchronizationreference signal for D2D communication can be efficiently transmitted ina wireless communication system.

The effects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages not described herein will be more clearly understood bypersons skilled in the art from the following detailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram illustrating a network structure of an EvolvedUniversal Mobile Telecommunications System (E-UMTS) which is an exampleof a wireless communication system.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a user equipment andE-UTRAN based on the 3GPP radio access network standard.

FIG. 3 is a diagram illustrating physical channels used in a 3GPP LTEsystem and a general method for transmitting a signal using the physicalchannels.

FIG. 4 is a diagram illustrating a structure of a radio frame used in anLTE system.

FIG. 5 illustrates a primary broadcast channel (P-BCH) and asynchronization channel (SCH) of an LTE system.

FIG. 6 illustrates a radio frame structure for transmission of asynchronization signal (SS).

FIG. 7 illustrates a secondary synchronization signal (SSS) generationscheme

FIG. 8 illustrates a resource grid of a DL slot.

FIG. 9 illustrates the structure of a DL subframe.

FIG. 10 illustrates the structure of a UL subframe in an LTE system.

FIG. 11 is a diagram conceptually illustrating D2D communication.

FIGS. 12 and 13 are diagrams referred to for describing the case inwhich an SR-UE is elected.

FIG. 14 is a diagram referred to for describing the case in which aspecific SR signal is linked to a specific resource region according tothe present invention.

FIG. 15 is a diagram referred to for describing an SR-UE contentionscheme according to the present invention.

FIGS. 16 and 17 illustrate the case in which an SR-relay UE (e.g., UE#11) that has received an SR signal of an SR-UE (e.g., SR-UE #1) relaysthe SR signal.

FIG. 18 illustrates the case in which an SR signal transmission resourceregion of an SR-relay UE and an SR signal transmission resource regionof an SR-UE are consecutively allocated.

FIG. 19 illustrates the case in which an SR signal transmission resourceregion of an SR-relay UE is allocated after a specific interval from anSR signal transmission resource region of an SR-UE.

FIG. 20 illustrates the case in which specific repeated resource regionsare reserved by an SR-UE and allocated to D2D communication.

FIG. 21 illustrates the case in which a random backoff completion timeof a D2DSS is not equal to the boundary of resource region durationsdivided by an SR-UE.

FIG. 22 is a diagram referred to for indicating a detailed example of aD2DSS signal.

FIG. 23 is a diagram referred to for describing a multi-hop relay of anSR.

FIGS. 24 and 25 are diagrams referred to for describing a resourceregion for relay signal transmission.

FIG. 26 illustrates the case in which an arbitrary UE receivesdistinguishable D2DSSs/PD2DSCHs from a plurality of SR-UEs.

FIG. 27 is a flowchart illustrating an SR-UE selection scheme when alevel value of a signal quality is applied.

FIG. 28 is a flowchart illustrating a scheme in which a UE operates asan SR-relay UE or an SR-UE.

FIG. 29 is a diagram referred to for describing a scheme of applying adifferential signal quality criterion according to a synchronizationpurpose of a UE.

FIG. 30 is a diagram referred to for describing the case in which aD2DSS/PD2DSCH is sequentially relayed when a maximum hop count is set.

FIG. 31 is a diagram referred to for describing the case in which aPD2DSCH becomes a single frequency network (SFN).

FIG. 32 is a diagram referred to for describing the case in which UEssupporting relay and UEs not supporting relay are mixed.

FIG. 33 is a diagram referred to for describing relay triggering when noSR-relay UEs are present.

FIG. 34 illustrates the case in which an attribute of a D2DSS with whicha UE is synchronized is changed due to mobility of the UE and a D2DSShaving a higher priority than a D2DSS with which the UE is synchronizedis detected.

FIG. 35 is a diagram referred to for describing reselection of a D2DSSin consideration of a hop count.

FIG. 36 illustrates the case in which a specific resource region isrepeatedly present in a D2DSS transmission period.

FIGS. 37 to 39 are diagrams illustrating the case in which transmissionresources are allocated at a specific interval according to the presentinvention.

FIG. 40 illustrates a BS and a UE which are applicable to an embodimentof the present invention.

FIG. 1 schematically illustrates an E-UMTS network structure as anexemplary wireless communication system.

BEST MODE

The following technology may be used for various wireless accesstechnologies such as CDMA (code division multiple access), FDMA(frequency division multiple access), TDMA (time division multipleaccess), OFDMA (orthogonal frequency division multiple access), andSC-FDMA (single carrier frequency division multiple access). The CDMAmay be implemented by the radio technology such as UTRA (universalterrestrial radio access) or CDMA2000. The TDMA may be implemented bythe radio technology such as global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). The OFDMA may be implemented by the radio technologysuch as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, andevolved UTRA (E-UTRA). The UTRA is a part of a universal mobiletelecommunications system (UMTS). A 3rd generation partnership projectlong term evolution (3GPP LTE) is a part of an evolved UMTS (E-UMTS)that uses E-UTRA, and adopts OFDMA in a downlink and SC-FDMA in anuplink. LTE-advanced (LTE-A) is an evolved version of the 3GPP LTE.

For clarification of the description, although the following embodimentswill be described based on the 3GPP LTE/LTE-A, it is to be understoodthat the technical spirits of the present invention are not limited tothe 3GPP LTE/LTE-A. Also, specific terminologies hereinafter used in theembodiments of the present invention are provided to assistunderstanding of the present invention, and various modifications may bemade in the specific terminologies within the range that they do notdepart from technical spirits of the present invention.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a user equipment andE-UTRAN based on the 3GPP radio access network standard. The controlplane means a passageway where control messages are transmitted, whereinthe control messages are used by the user equipment and the network tomanage call. The user plane means a passageway where data generated inan application layer, for example, voice data or Internet packet dataare transmitted.

A physical layer as the first layer provides an information transferservice to an upper layer using a physical channel. The physical layeris connected to a medium access control (MAC) layer via a transportchannel, wherein the medium access control layer is located above thephysical layer. Data are transferred between the medium access controllayer and the physical layer via the transport channel. Data aretransferred between one physical layer of a transmitting side and theother physical layer of a receiving side via the physical channel. Thephysical channel uses time and frequency as radio resources. In moredetail, the physical channel is modulated in accordance with anorthogonal frequency division multiple access (OFDMA) scheme in adownlink, and is modulated in accordance with a single carrier frequencydivision multiple access (SC-FDMA) scheme in an uplink.

A medium access control (MAC) layer of the second layer provides aservice to a radio link control (RLC) layer above the MAC layer via alogical channel. The RLC layer of the second layer supports reliabledata transmission. The RLC layer may be implemented as a functionalblock inside the MAC layer. In order to effectively transmit data usingIP packets such as IPv4 or IPv6 within a radio interface having a narrowbandwidth, a packet data convergence protocol (PDCP) layer of the secondlayer performs header compression to reduce the size of unnecessarycontrol information.

A radio resource control (RRC) layer located on the lowest part of thethird layer is defined in the control plane only. The RRC layer isassociated with configuration, re-configuration and release of radiobearers (‘RBs’) to be in charge of controlling the logical, transportand physical channels. In this case, the RB means a service provided bythe second layer for the data transfer between the user equipment andthe network. To this end, the RRC layers of the user equipment and thenetwork exchange RRC message with each other. If the RRC layer of theuser equipment is RRC connected with the RRC layer of the network, theuser equipment is in an RRC connected mode. If not so, the userequipment is in an RRC idle mode. A non-access stratum (NAS) layerlocated above the RRC layer performs functions such as sessionmanagement and mobility management.

One cell constituting a base station eNB is set to one of bandwidths of1.4, 3.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to several user equipments. At this time, differentcells may be set to provide different bandwidths.

As downlink transport channels carrying data from the network to theuser equipment, there are provided a broadcast channel (BCH) carryingsystem information, a paging channel (PCH) carrying paging message, anda downlink shared channel (SCH) carrying user traffic or controlmessages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted via the downlink SCH or anadditional downlink multicast channel (MCH). Meanwhile, as uplinktransport channels carrying data from the user equipment to the network,there are provided a random access channel (RACH) carrying an initialcontrol message and an uplink shared channel (UL-SCH) carrying usertraffic or control message. As logical channels located above thetransport channels and mapped with the transport channels, there areprovided a broadcast control channel (BCCH), a paging control channel(PCCH), a common control channel (CCCH), a multicast control channel(MCCH), and a multicast traffic channel (MTCH).

FIG. 3 is a diagram illustrating physical channels used in a 3GPP LTEsystem and a general method for transmitting a signal using the physicalchannels.

The user equipment performs initial cell search such as synchronizingwith the base station when it newly enters a cell or the power is turnedon at step S301. To this end, the user equipment synchronizes with thebase station by receiving a primary synchronization channel (P-SCH) anda secondary synchronization channel (S-SCH) from the base station, andacquires information such as cell ID, etc. Afterwards, the userequipment may acquire broadcast information within the cell by receivinga physical broadcast channel (PBCH) from the base station. Meanwhile,the user equipment may identify a downlink channel status by receiving adownlink reference signal (DL RS) at the initial cell search step.

The user equipment which has finished the initial cell search mayacquire more detailed system information by receiving a physicaldownlink shared channel (PDSCH) in accordance with a physical downlinkcontrol channel (PDCCH) and information carried in the PDCCH at stepS302.

Afterwards, the user equipment may perform a random access procedure(RACH) such as steps S303 to S306 to complete access to the basestation. To this end, the user equipment may transmit a preamble througha physical random access channel (PRACH) (S303), and may receive aresponse message to the preamble through the PDCCH and the PDSCHcorresponding to the PDCCH (S304). In case of a contention based RACH,the user equipment may perform a contention resolution procedure such astransmission (S305) of additional physical random access channel andreception (S306) of the physical downlink control channel and thephysical downlink shared channel corresponding to the physical downlinkcontrol channel.

The user equipment which has performed the aforementioned steps mayreceive the physical downlink control channel (PDCCH)/physical downlinkshared channel (PDSCH) (S307) and transmit a physical uplink sharedchannel (PUSCH) and a physical uplink control channel (PUCCH) (S308), asa general procedure of transmitting uplink/downlink signals. Controlinformation transmitted from the user equipment to the base station willbe referred to as uplink control information (UCI). The UCI includesHARQ ACK/NACK (Hybrid Automatic Repeat and reQuestAcknowledgement/Negative-ACK), SR (Scheduling Request), CSI (ChannelState Information), etc. In this specification, the HARQ ACK/NACK willbe referred to as HARQ-ACK or ACK/NACK (A/N). The HARQ-ACK includes atleast one of positive ACK (simply, referred to as ACK), negative ACK(NACK), DTX and NACK/DTX. The CSI includes CQI (Channel QualityIndicator), PMI (Precoding Matrix Indicator), RI (Rank Indication), etc.Although the UCI is generally transmitted through the PUCCH, it may betransmitted through the PUSCH if control information and traffic datashould be transmitted at the same time. Also, the user equipment maynon-periodically transmit the UCI through the PUSCH in accordance withrequest/command of the network.

FIG. 4 is a diagram illustrating a structure of a radio frame used in anLTE system.

Referring to FIG. 4, in a cellular OFDM radio packet communicationsystem, uplink/downlink data packet transmission is performed in a unitof subframe, wherein one subframe is defined by a given time intervalthat includes a plurality of OFDM symbols. The 3GPP LTE standardsupports a type 1 radio frame structure applicable to frequency divisionduplex (FDD) and a type 2 radio frame structure applicable to timedivision duplex (TDD).

FIG. 4(a) is a diagram illustrating a structure of a type 1 radio frame.The downlink radio frame includes 10 subframes, each of which includestwo slots in a time domain. A time required to transmit one subframewill be referred to as a transmission time interval (TTI). For example,one subframe may have a length of 1 ms, and one slot may have a lengthof 0.5 ms. One slot includes a plurality of OFDM symbols in a timedomain and a plurality of resource blocks (RB) in a frequency domain.Since the 3GPP LTE system uses OFDM in a downlink, OFDM symbolsrepresent one symbol interval. The OFDM symbol may be referred to asSC-FDMA symbol or symbol interval. The resource block (RB) as a resourceallocation unit may include a plurality of continuous subcarriers in oneslot.

The number of OFDM symbols included in one slot may be varied dependingon configuration of a cyclic prefix (CP). Examples of the CP include anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be 7. If the OFDM symbols are configured by the extended CP,since the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is smaller than that of OFDM symbols incase of the normal CP. For example, in case of the extended CP, thenumber of OFDM symbols included in one slot may be 6. If a channel stateis unstable like the case where the user equipment moves at high speed,the extended CP may be used to reduce inter-symbol interference.

If the normal CP is used, since one slot includes seven OFDM symbols,one subframe includes 14 OFDM symbols. At this time, first maximum threeOFDM symbols of each subframe may be allocated to a physical downlinkcontrol channel (PDCCH), and the other OFDM symbols may be allocated toa physical downlink shared channel (PDSCH).

FIG. 4(b) is a diagram illustrating a structure of a type 2 radio frame.The type 2 radio frame includes two half frames, each of which includesfour general subframes, which include two slots, and a special subframewhich includes a downlink pilot time slot (DwPTS), a guard period (GP),and an uplink pilot time slot (UpPTS).

In the special subframe, the DwPTS is used for initial cell search,synchronization or channel estimation at the user equipment. The UpPTSis used for channel estimation at the base station and uplinktransmission synchronization of the user equipment. In other words, theDwPTS is used for downlink transmission, whereas the UpPTS is used foruplink transmission. Especially, the UpPTS is used for PRACH preamble orSRS transmission. Also, the guard period is to remove interferenceoccurring in the uplink due to multipath delay of downlink signalsbetween the uplink and the downlink.

Configuration of the special subframe is defined in the current 3GPPstandard document as illustrated in Table 1 below. Table 1 illustratesthe DwPTS and the UpPTS in case of T_(s)=1/(15000×2048), and the otherregion is configured for the guard period.

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS UpPTS Normal Extended Normal Extended Special subframecyclic prefix cyclic prefix cyclic prefix cyclic prefix configurationDwPTS in uplink in uplink DwPTS in uplink in uplink 0  6592 · T_(s) 2192· T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 ·T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600· T_(s) 4 26336 · T_(s)  7680 · T_(s) 5  6592 · T_(s) 4384 · T_(s) 5120· T_(s) 20480 · T_(s) 4384 · T_(s) 5120 · T_(s) 6 19760 · T_(s) 23040 ·T_(s) 7 21952 · T_(s) 12800 · T_(s) 8 24144 · T_(s) — — — 9 13168 ·T_(s) — — —

In the meantime, the structure of the type 2 radio frame, that is,uplink/downlink configuration (UL/DL configuration) in the TDD system isas illustrated in Table 2 below.

TABLE 2 Downlink- to-Uplink Uplink- Switch- downlink point Subframenumber configuration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U DS U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  DS U U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D DD D 6 5 ms D S U U U D S U U D

In the above Table 2, D means the downlink subframe, U means the uplinksubframe, and S means the special subframe. Also, Table 2 alsoillustrates a downlink-uplink switching period in the uplink/downlinksubframe configuration of each system.

The structure of the aforementioned radio frame is only exemplary, andvarious modifications may be made in the number of subframes included inthe radio frame, the number of slots included in the subframe, or thenumber of symbols included in the slot.

FIG. 5 illustrates a primary broadcast channel (P-BCH) and asynchronization channel (SCH) of an LTE system. The SCH includes a P-SCHand an S-SCH. A primary synchronization signal (PSS) is transmitted overthe P-SCH and a secondary synchronization signal (SSS) is transmittedover an S-SCH.

Referring to FIG. 5, in frame structure type-1 (i.e., FDD), the P-SCH islocated in each of slot #0 (i.e., the first slot of subframe #0) andslot #10 (i.e., the first slot of subframe #5) in every radio frame. TheS-SCH is located on an OFDM symbol immediately prior to the last OFDMsymbol of each of slot #0 and slot #10 in every radio frame. The S-SCHand the P-SCH are located on adjacent OFDM symbols. In frame structuretype-2 (i.e., TDD), the P-SCH is transmitted on the third OFDM symbol ofeach of subframes #1 and #6 and the S-SCH is located on the last OFDMsymbol of each of slot #1 (i.e., the second slot of subframe #0) andslot #11 (i.e., the second slot of subframe #5). The P-BCH istransmitted in every four radio frame regardless of a frame structuretype and is transmitted using the first to fourth OFDM symbols of thesecond slot of subframe #0.

The P-SCH is transmitted using 72 subcarriers (10 subcarriers beingreserved and 62 subcarriers carrying PSS) based on a direct current (DC)subcarrier on a corresponding OFDM symbol. The S-SCH is transmittedusing 72 subcarriers (10 subcarriers being reserved and 62 subcarrierscarrying an SSS) based on a DC subcarrier on a corresponding OFDMsymbol. The P-BCH is mapped to four OFDM symbols and 72 subcarriersbased on a DC subcarrier, in one subframe.

FIG. 6 illustrates a radio frame structure for transmission of asynchronization signal (SS). Specifically, FIG. 6 illustrates a radioframe structure for transmission of an SS and a PBCH in frequencydivision duplex (FDD), wherein FIG. 6(a) illustrates transmissionlocations of an SS and a PBCH in a radio frame configured as a normalcyclic prefix (CP) and FIG. 6(b) illustrates transmission locations ofan SS and a PBCH in a radio frame configured as an extended CP.

If a UE is powered on or newly enters a cell, the UE performs an initialcell search procedure of acquiring time and frequency synchronizationwith the cell and detecting a physical cell identity of the cell. Tothis end, the UE may establish synchronization with the eNB by receivingsynchronization signals, e.g. a primary synchronization signal (PSS) anda secondary synchronization signal (SSS), from the eNB and obtaininformation such as a cell identity (ID).

An SS will be described in more detail with reference to FIG. 6. An SSis categorized into a PSS and an SSS. The PSS is used to acquiretime-domain synchronization of OFDM symbol synchronization, slotsynchronization, etc. and/or frequency-domain synchronization and theSSS is used to acquire frame synchronization, a cell group ID, and/or CPconfiguration of a cell (i.e. information as to whether a normal CP isused or an extended CP is used). Referring to FIG. 6, each of a PSS andan SSS is transmitted on two OFDM symbols of every radio frame. Morespecifically, SSs are transmitted in the first slot of subframe 0 andthe first slot of subframe 5, in consideration of a global system formobile communication (GSM) frame length of 4.6 ms for facilitation ofinter-radio access technology (inter-RAT) measurement. Especially, a PSSis transmitted on the last OFDM symbol of the first slot of subframe 0and on the last OFDM symbol of the first slot of subframe 5 and an SSSis transmitted on the second to last OFDM symbol of the first slot ofsubframe 0 and on the second to last OFDM symbol of the first slot ofsubframe 5. A boundary of a corresponding radio frame may be detectedthrough the SSS. The PSS is transmitted on the last OFDM symbol of acorresponding slot and the SSS is transmitted on an OFDM symbolimmediately before an OFDM symbol on which the PSS is transmitted. Atransmit diversity scheme of an SS uses only a single antenna port andstandards therefor are not separately defined. That is, a single antennaport transmission scheme or a transmission scheme transparent to a UE(e.g. precoding vector switching (PVS), time switched transmit diversity(TSTD), or cyclic delay diversity (CDD)) may be used for transmitdiversity of an SS.

An SS may represent a total of 504 unique physical layer cell IDs by acombination of 3 PSSs and 168 SSSs. In other words, the physical layercell IDs are divided into 168 physical layer cell ID groups eachincluding three unique IDs so that each physical layer cell ID is a partof only one physical layer cell ID group. Accordingly, a physical layercell ID N^(cell) _(ID) (=3N⁽¹⁾ _(ID)+N⁽²⁾ _(ID)) is uniquely defined asnumber N⁽¹⁾ _(ID) in the range of 0 to 167 indicating a physical layercell ID group and number N⁽²⁾ID from 0 to 2 indicating the physicallayer ID in the physical layer cell ID group. A UE may be aware of oneof three unique physical layer IDs by detecting the PSS and may be awareof one of 168 physical layer cell IDs associated with the physical layerID by detecting the SSS. A length-63 Zadoff-Chu (ZC) sequence is definedin the frequency domain and is used as the PSS. As an example, the ZCsequence may be defined by the following equation.

$\begin{matrix}{{d_{u}(n)} = ^{{- j}\frac{\pi \; {{un}{({n + 1})}}}{N_{ZC}}}} & \lbrack{Equation}\rbrack\end{matrix}$

where N_(ZC)=63 and a sequence element corresponding to a DC subcarrier,n=31, is punctured.

The PSS is mapped to 6 RBs (=72 subcarriers) near a center frequency.Among the 72 subcarriers, 9 remaining subcarriers always carry a valueof 0 and serve as elements facilitating filter design for performingsynchronization. To define a total of three PSSs, u=24, 29, and 34 areused in Equation 1. Since u=24 and u=34 have a conjugate symmetryrelation, two correlations may be simultaneously performed. Here,conjugate symmetry indicates the relationship of the following Equation.

d _(u)(n)=(−1)^(n)(d _(N) _(ZC) _(−u)(n))*,when N _(ZC) is even number

d _(u)(n)=(d _(N) _(ZC) _(−u)(n))*,when N _(ZC) is odd number  [Equation2]

A one-shot correlator for u=29 and u=34 may be implemented using thecharacteristics of conjugate symmetry. The entire amount of calculationcan be reduced by about 33.3% as compared with the case withoutconjugate symmetry.

In more detail, a sequence d(n) used for a PSS is generated from afrequency-domain ZC sequence as follows.

$\begin{matrix}{{d_{u}(n)} = \left\{ \begin{matrix}^{{- j}\frac{\pi \; {{un}{({n + 1})}}}{63}} & {{n = 0},1,\ldots \mspace{14mu},30} \\^{{- j}\frac{\pi \; {u{({n + 1})}}{({n + 2})}}{63}} & {{n = 31},32,\ldots \mspace{14mu},61}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Where a ZC root sequence index u is given by the following table.

TABLE 3 N⁽²⁾ _(ID) Root index u 0 25 1 29 2 34

Referring to FIG. 6, upon detecting a PSS, a UE may discern that acorresponding subframe is one of subframe 0 and subframe 5 because thePSS is transmitted every 5 ms but the UE cannot discern whether thesubframe is subframe 0 or subframe 5. Accordingly, the UE cannotrecognize the boundary of a radio frame only by the PSS. That is, framesynchronization cannot be acquired only by the PSS. The UE detects theboundary of a radio frame by detecting an SSS which is transmitted twicein one radio frame with different sequences.

FIG. 7 illustrates an SSS generation scheme. Specifically, FIG. 7illustrates a relationship of mapping of two sequences in a logicaldomain to sequences in a physical domain.

A sequence used for the SSS is an interleaved concatenation of twolength-31 m-sequences and the concatenated sequence is scrambled by ascrambling sequence given by a PSS. Here, an m-sequence is a type of apseudo noise (PN) sequence.

Referring to FIG. 7, if two m-sequences used for generating an SSS codeare S1 and S2, then S1 and S2 are obtained by scrambling two differentPSS-based sequences to the SSS. In this case, S1 and S2 are scrambled bydifferent sequences. A PSS-based scrambling code may be obtained bycyclically shifting an m-sequence generated from a polynomial of x⁵+x³+1and 6 sequences are generated by cyclic shift of the m-sequenceaccording to an index of a PSS. Next, S2 is scrambled by an S1-basedscrambling code. The S1-based scrambling code may be obtained bycyclically shifting an m-sequence generated from a polynomial ofx⁵+x⁴+x²+x¹+1 and 8 sequences are generated by cyclic shift of them-sequence according to an index of S1. The SSS code is swapped every 5ms, whereas the PSS-based scrambling code is not swapped. For example,assuming that an SSS of subframe 0 carries a cell group ID by acombination of (S1, S2), an SSS of subframe 5 carries a sequence swappedas (S2, S1). Hence, a boundary of a radio frame of 10 ms can bediscerned. In this case, the used SSS code is generated from apolynomial of x⁵+x²+1 and a total of 31 codes may be generated bydifferent cyclic shifts of an m-sequence of length-31.

A combination of two length-31 m-sequences for defining the SSS isdifferent in subframe 0 and subframe 5 and a total of 168 cell group IDsare expressed by a combination of the two length-31 m-sequences. Them-sequences used as sequences of the SSS have a robust property in afrequency selective environment. In addition, since the m-sequences canbe transformed by high-speed m-sequence transform using fast Hadamardtransform, if the m-sequences are used as the SSS, the amount ofcalculation necessary for a UE to interpret the SSS can be reduced.Since the SSS is configured by two short codes, the amount ofcalculation of the UE can be reduced.

Generation of the SSS will now be described in more detail. A sequenced(0), . . . , d(61) used for the SSS is an interleaved concatenation oftwo length-31 binary sequences. The concatenated sequence is scrambledby a sequence given by the PSS.

A combination of two length-31 sequences for defining the PSS becomesdifferent in subframe 0 and subframe 5 as follows.

$\begin{matrix}{\mspace{79mu} {{d\left( {2n} \right)} = \left\{ {{\begin{matrix}{{s_{0}^{(m_{0})}(n)}{c_{0}(n)}} & {{in}\mspace{14mu} {subframe}\mspace{14mu} 0} \\{{s_{1}^{(m_{1})}(n)}{c_{0}(n)}} & {{in}\mspace{14mu} {subframe}\mspace{14mu} 5}\end{matrix}{d\left( {{2n} + 1} \right)}} = \left\{ \begin{matrix}{{s_{1}^{(m_{1})}(n)}{c_{1}(n)}{z_{1}^{(m_{0})}(n)}} & {{in}\mspace{14mu} {subframe}\mspace{14mu} 0} \\{{s_{0}^{(m_{0})}(n)}{c_{1}(n)}{z_{1}^{(m_{1})}(n)}} & {{in}\mspace{14mu} {subframe}\mspace{14mu} 5}\end{matrix} \right.} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, 0≦n≦30. Indices m₀ and m₁ are derived from aphysical-layer cell-identity group N⁽¹⁾ _(ID) as follows.

$\begin{matrix}{{m_{0} = {m^{\prime}{mod}\mspace{14mu} 31}}{m_{1} = {\left( {m_{0} + \left\lfloor {m^{\prime}/31} \right\rfloor + 1} \right){mod}\mspace{14mu} 31}}{{m^{\prime} = {N_{ID}^{(1)} + {{q\left( {q + 1} \right)}/2}}},{q = \left\lfloor \frac{N_{ID}^{(1)} + {{q^{\prime}\left( {q^{\prime} + 1} \right)}/2}}{30} \right\rfloor},{q^{\prime} = \left\lfloor {N_{ID}^{(1)}/30} \right\rfloor}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

The output of Equation 5 is listed in Table 4 that follows Equation 11.

Two sequences s^((m0)) ₀(n) and s^((m1)) ₁(n) are defined as twodifferent cyclic shifts of an m-sequence s(n).

s ₀ ^((m) ⁰ ⁾(n)=s((n+m ₀)mod 31)

s ₁ ^((m) ¹ ⁾(n)=s((n+m ₁)mod 31)

where s(i)=1−2x(i) (0≦i≦30) is defined by the following equation withinitial conditions x(0)=0, x(1)=0, x(2), x(3)=0, x(4)=1.

x(ī+5)=(x(ī+3)+x(ī))mod 2,0≦ī≦25  [Equation 7]

Two scrambling sequences c₀(n) and c₁(n) depend on the PSS and aredefined by two different cyclic shifts of an m-sequence c(n).

c ₀(n)=c((n+N _(ID) ⁽²⁾)mod 31)

c ₁(n)=c((n+N _(ID) ⁽²⁾+3)mod 31)[Equation 8]

where N⁽²⁾ _(ID)ε{0, 1, 2} is a physical-layer identity within aphysical-layer cell identity group N⁽¹⁾ _(ID) and c(i)=1−2x(i) (0≦i≦30)is defined by the following equation with initial conditions x(0)=0,x(1)=0, x(2), x(3)=0, x(4)=1.

x(ī+5)=(x(ī+3)+x(ī))mod 2,0≦ī≦25  [Equation 9]

Scrambling sequences Z^((m0)1)(n) and Z^((m1)1)(n) are defined by acyclic shift of an m-sequence z(n).

z ₁ ^((m) ⁰ ⁾(n)=z((n+(m ₀ mod 8))mod 31)

z ₁ ^((m) ¹ ⁾(n)=z((n+(m ₁ mod 8))mod 31)

where m₀ and m₁ are obtained from Table 4 that follows Equation 11 andz(i)=1−2x(i) (0≦i≦30) is defined by the following equation with initialconditions x(0)=0, x(1)=0, x(2), x(3)=0, x(4)=1.

x(ī+5)=(x(ī+4)+x(ī+2)+x(ī+1)+x(ī))mod 2,0≦ī≦25  [Equation 11]

TABLE 4 N⁽¹⁾ _(ID) m₀ m₁ 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 88 8 9 9 9 10 10 10 11 11 11 12 12 12 13 13 13 14 14 14 15 15 15 16 16 1617 17 17 18 18 18 19 19 19 20 20 20 21 21 21 22 22 22 23 23 23 24 24 2425 25 25 26 26 26 27 27 27 28 28 28 29 29 29 30 30 0 2 31 1 3 32 2 4 333 5 34 4 6 35 5 7 36 6 8 37 7 9 38 8 10 39 9 11 40 10 12 41 11 13 42 1214 43 13 15 44 14 16 45 15 17 46 16 18 47 17 19 48 18 20 49 19 21 50 2022 51 21 23 52 22 24 53 23 25 54 24 26 55 25 27 56 26 28 57 27 29 58 2830 59 0 3 60 1 4 61 2 5 62 3 6 63 4 7 64 5 8 65 6 9 66 7 10 67 8 11 68 912 69 10 13 70 11 14 71 12 15 72 13 16 73 14 17 74 15 18 75 16 19 76 1720 77 18 21 78 19 22 79 20 23 80 21 24 81 22 25 82 23 26 83 24 27 84 2528 85 26 29 86 27 30 87 0 4 88 1 5 89 2 6 90 3 7 91 4 8 92 5 9 93 6 1094 7 11 95 8 12 96 9 13 97 10 14 98 11 15 99 12 16 100 13 17 101 14 18102 15 19 103 16 20 104 17 21 105 18 22 106 19 23 107 20 24 108 21 25109 22 26 110 23 27 111 24 28 112 25 29 113 26 30 114 0 5 115 1 6 116 27 117 3 8 118 4 9 119 5 10 120 6 11 121 7 12 122 8 13 123 9 14 124 10 15125 11 16 126 12 17 127 13 18 128 14 19 129 15 20 130 16 21 131 17 22132 18 23 133 19 24 134 20 25 135 21 26 136 22 27 137 23 28 138 24 29139 25 30 140 0 6 141 1 7 142 2 8 143 3 9 144 4 10 145 5 11 146 6 12 1477 13 148 8 14 149 9 15 150 10 16 151 11 17 152 12 18 153 13 19 154 14 20155 15 21 156 16 22 157 17 23 158 18 24 159 19 25 160 20 26 161 21 27162 22 28 163 23 29 164 24 30 165 0 7 166 1 8 167 2 9 — — — — — —

A UE, which has demodulated a DL signal by performing a cell searchprocedure using an SSS and determined time and frequency parametersnecessary for transmitting a UL signal at an accurate time, cancommunicate with an eNB only after acquiring system informationnecessary for system configuration of the UE from the eNB.

The system information is configured by a master information block (MIB)and system information blocks (SIBs). Each SIB includes a set offunctionally associated parameters and is categorized into an MIB, SIBType 1 (SIB1), SIB Type 2 (SIB2), and SIB3 to SIB8 according to includedparameters. The MIB includes most frequency transmitted parameters whichare essential for initial access of the UE to a network of the eNB. SIB1includes parameters needed to determine if a specific cell is suitablefor cell selection, as well as information about time-domain schedulingof the other SIBs.

The UE may receive the MIB through a broadcast channel (e.g. a PBCH).The MIB includes DL bandwidth (BW), PHICH configuration, and a systemframe number SFN. Accordingly, the UE can be explicitly aware ofinformation about the DL BW, SFN, and PHICH configuration by receivingthe PBCH. Meanwhile, information which can be implicitly recognized bythe UE through reception of the PBCH is the number of transmit antennaports of the eNB. Information about the number of transmit antennas ofthe eNB is implicitly signaled by masking (e.g. XOR operation) asequence corresponding to the number of transmit antennas to a 16-bitcyclic redundancy check (CRC) used for error detection of the PBCH.

The PBCH is mapped to four subframes during 40 ms. The time of 40 ms isblind-detected and explicit signaling about 40 ms is not separatelypresent. In the time domain, the PBCH is transmitted on OFDM symbols 0to 3 of slot 1 in subframe 0 (the second slot of subframe 0) of a radioframe.

In the frequency domain, a PSS/SSS and a PBCH are transmitted only in atotal of 6 RBs, i.e. a total of 72 subcarriers, irrespective of actualsystem BW, wherein 3 RBs are on the left and the other 3 RBs are on theright centering on a DC subcarrier on corresponding OFDM symbols.Therefore, the UE is configured to detect or decode the SS and the PBCHirrespective of DL BW configured for the UE.

After initial cell search, a UE which has accessed a network of an eNBmay acquire more detailed system information by receiving a PDCCH and aPDSCH according to information carried on the PDCCH. The UE which hasperformed the above-described procedure may perform reception of aPDCCH/PDSCH and transmission of a PUSCH/PUCCH as a normal UL/DL signaltransmission procedure.

FIG. 8 illustrates a resource grid of a DL slot.

Referring to FIG. 8, a DL slot includes N^(DL) _(symb) OFDM symbols inthe time domain and N^(DL) _(RB) RBs in the frequency domain. Each RBincludes N^(RB) _(sc) subcarriers and thus the DL slot includes N_(RB)^(DL)×N_(sc) ^(RB) subcarriers in the frequency domain. Although FIG. 8illustrates the case in which a DL slot includes 7 OFDM symbols and anRB includes 12 subcarriers, the present invention is not limitedthereto. For example, the number of OFDM symbols included in the DL slotmay differ according to CP length.

Each element on the resource grid is referred to as a resource element(RE). One RE is indicated by one OFDM symbol index and one subcarrierindex. One RB includes N_(symb) ^(DL)×N_(sc) ^(RB) REs. The number ofRBs, N_(RB) ^(DL), included in a DL slot depends on DL bandwidthconfigured in a cell.

FIG. 9 illustrates the structure of a DL subframe.

Referring to FIG. 9, up to three (or four) OFDM symbols at the start ofthe first slot of a DL subframe are used as a control region to whichcontrol channels are allocated and the other OFDM symbols of the DLsubframe are used as a data region to which a PDSCH is allocated. DLcontrol channels defined for an LTE system include a physical controlformat indicator channel (PCFICH), a physical downlink control channel(PDCCH), and a physical hybrid ARQ indicator channel (PHICH). The PCFICHis transmitted in the first OFDM symbol of a subframe, carryinginformation about the number of OFDM symbols used for transmission ofcontrol channels in the subframe. The PHICH delivers a HARQ ACK/NACKsignal as a response to UL transmission.

Control information carried on the PDCCH is called downlink controlinformation (DCI). The DCI transports resource allocation informationand other control information for a UE or a UE group. For example, theDCI includes DL/UL scheduling information, UL transmit (Tx) powercontrol commands, etc.

The PDCCH delivers information about resource allocation and a transportformat for a downlink shared channel (DL-SCH), information aboutresource allocation and a transport format for an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a random access responsetransmitted on the PDSCH, a set of transmit power control commands forindividual UEs of a UE group, Tx power control commands, voice overInternet protocol (VoIP) activation indication information, etc. Aplurality of PDCCHs may be transmitted in the control region. A UE maymonitor a plurality of PDCCHs. A PDCCH is transmitted on an aggregate ofone or more consecutive control channel elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE corresponds to a plurality ofresource element groups (REGs). The format of a PDCCH and the number ofavailable bits for the PDCCH are determined according to the number ofCCEs. An eNB determines a PDCCH format according to DCI transmitted to aUE and attaches a cyclic redundancy check (CRC) to control information.The CRC is masked with an identifier (ID) (e.g. a radio networktemporary identifier (RNTI)) according to the owner or use of the PDCCH.If the PDCCH is destined for a specific UE, the CRC may be masked with acell-RNTI (C-RNTI) of the UE. If the PDCCH carries a paging message, theCRC may be masked with a paging ID (P-RNTI). If the PDCCH carries systeminformation (particularly, a system information block (SIB)), the CRCmay be masked with a system information RNTI (SI-RNTI). If the PDCCH isdesignated as a random access response, the CRC may be masked with arandom access-RNTI (RA-RNTI).

FIG. 10 illustrates the structure of a UL subframe in an LTE system.

Referring to FIG. 10, a UL subframe includes a plurality of (e.g. 2)slots. A slot may include a different number of SC-FDMA symbolsaccording to CP length. The UL subframe is divided into a control regionand a data region in the frequency domain. The data region includes aPUSCH to transmit a data signal such as voice and the control regionincludes a PUCCH to transmit UCI. The PUCCH occupies a pair of RBs atboth ends of the data region in the frequency domain and the RB pairfrequency-hops over a slot boundary.

The PUCCH may deliver the following control information.

-   -   SR: SR is information requesting UL-SCH resources and is        transmitted using on-off keying (OOK).    -   HARQ ACK/NACK: HARQ ACK/NACK is a response signal to a DL data        packet received on a PDSCH, indicating whether the DL data        packet has been successfully received. 1-bit ACK/NACK is        transmitted as a response to a single DL codeword and 2-bit        ACK/NACK is transmitted as a response to two DL codewords.    -   CSI: CSI is feedback information regarding a DL channel. CSI        includes a CQI and multiple input multiple output (MIMO)-related        feedback information includes an RI, a PMI, a precoding type        indicator (PTI), etc. The CSI occupies 20 bits per subframe.

The amount of UCI that the UE may transmit in a subframe depends on thenumber of SC-FDMA symbols available for transmission of controlinformation. The remaining SC-FDMA symbols except for SC-FDMA symbolsallocated to RSs in a subframe are available for transmission of controlinformation. If the subframe carries an SRS, the last SC-FDMA symbol ofthe subframe is also excluded in transmitting the control information.The RSs are used for coherent detection of the PUCCH.

FIG. 12 is a diagram conceptually illustrating D2D communication. FIG.12(a) illustrates a conventional eNB-based communication scheme in whicha first UE UE1 transmits data to an eNB on UL and the eNB transmits thedata from the first UE UE1 to a second UE2 on DL.

FIG. 12(b) illustrates a UE-to-UE communication scheme as exemplary D2Dcommunication in which data is exchanged between UEs without passingthrough the eNB. Such a link directly established between devices may bereferred to as a D2D link. D2D communication has the advantage ofreducing latency relative to the conventional eNB-based communicationscheme and requiring fewer radio resources.

Although D2D communication is a scheme of supporting communicationbetween devices (or UEs) without passing through the eNB, sinceresources of an existing wireless communication system (e.g., 3GPPLTE/LTE-A) are reused to perform D2D communication, D2D communicationshould not create interference or disturbance with the existing wirelesscommunication system. In the same context, it is also important tominimize interference that a UE or an eNB operating in the existingwireless communication system create during D2D.

Based on the above-described discussion, a method will be described forelecting/determining a UE that can be a synchronization reference whenthere is no synchronization reference such as an eNB in performing D2Dcommunication between UEs proposed in the present invention.

In the present invention, synchronization may include OFDM symbolsynchronization and frequency synchronization and, in the case of usinga frame structure, also include frame synchronization. Typically, asynchronization signal in a 3GPP LTE system may refer to a signal suchas a PSS/SSS that an eNB transmits. An entity corresponding to thesynchronization reference may be an entity that transmits theabove-described synchronization signal and may be, for example, an eNBin the 3GPP LTE system.

In D2D communication, when a plurality of UEs is synchronized with eachother, radio resources can be effectively used. If UEs are locatedwithin coverage of an eNB, the UEs may acquire synchronization of eachother by being synchronized with time and frequency of the eNB. However,at least some UEs performing D2D communication are out of coverage ofthe eNB, an entity, such as the eNB, capable of providing a commonsynchronization reference to UEs performing D2D communication is notpresent any longer.

Therefore, as a method used for synchronization between UEs, a specificUE may become a synchronization reference to provide synchronizationcapable of being commonly applied to UEs located within coveragethereof. A UE that provides a synchronization reference signal isdefined as a synch-reference signal (RS) UE, a synchronization reference(SR)-UE, or a cluster head and a set of UEs synchronized with the SR-UEis defined as a (D2D) cluster.

In the above-described case, a method as to which UE is to be elected asan SR-UE from among D2D UEs is needed. In electing the SR-UE, it isnecessary to configure a network type in which too many unsynchronizedclusters do not overlap each other and, at the same time, to synchronizeall UEs in a normal UE distribution with a specific cluster.

Therefore, a UE may determine whether the UE will be an SR-UE or the UEwill be synchronized with another SR-UE according to the following steps1 to 4 and perform an operation defined in each step.

-   -   SR-UE determination step 1: The UE scans for an SR signal during        a scanning time period.    -   SR-UE determination step 2: The UE determines whether to        participate in contention for becoming an SR-UE. If the UE does        not participate in contention, the UE performs SR-UE        determination step 3. However, if the UE participates in        contention for being elected as the SR-UE, the UE proceeds to        SR-UE determination step 4.    -   SR-UE determination step 3: The UE joins a cluster of one or        more SR-UEs. For transmission in a specific resource region, the        UE uses an SR signal linked to the corresponding resource        region.    -   SR-UE determination step 4: The UE participates in contention        for SR signal transmission. A scheme is used in which random        backoff is set and a UE, a random backoff interval of which ends        first, wins contention.

In this case, a resource corresponding to a specific time period may bereserved for one SR. The size/amount of the time period may bepredetermined and may be transmitted through a SR signal. In otherwords, information about the size of the time period may be embedded inthe SR signal or may be included in a message region designated throughthe SR signal.

In determining whether to participate in contention for becoming theSR-UE (i.e., SR-UE determination step 2), the strength of a received SRsignal may be considered. For example, a predetermined threshold valuemay be set. If the strength of the received SR signal is greater thanthe threshold value, the UE may not participate in contention and, ifthe strength of the received SR signal is less than the threshold value,the UE may operate to participate in contention.

FIG. 12 is a diagram referred to for describing the case in which anSR-UE is elected.

In FIG. 12, SR-UE #1 first scans an SR signal. If SR-UE #1 finds no SRsignal in this SR signal scanning procedure (i.e., SR-UE determinationstep 1), SR-UE #1 determines to participate in contention for becomingan SR-UE (i.e., SR-UE determination step 2). Therefore, SR-UE #1 mayselect a random backoff number as in SR-UE determination step 4described above, perform as many backoff procedures as backoff slots (inthis case, it may be assumed that the backoff slots are generally lessthan a subframe duration and greater than a symbol duration), andreserves a specific resource period to perform D2D communicationtogether with UEs (UE #10 and UE #11) synchronized therewith. Further,if the resource reservation period is ended, SR-UE #1 may participate incontention again.

In this case, upon receiving an SR signal less than a threshold value,transmitted by SR-UE #1, a UE may perform SR-UE determination steps 1 to4 described above at a timing thereof and then form a cluster thereof byindependently performing contention with SR-UE #1. For example, in FIG.12(a) and FIG. 12 (b), because the SR signal received from SR-UE #1 isless than the threshold value, SR-UE #2 may perform the SR-UEdetermination step for being elected as an SR-UE according to a timingthereof and perform D2D communication by forming a cluster with UE #21and UE #22.

Alternatively, as illustrated in FIG. 12(c), a UE that receives an SRsignal less than the threshold value, transmitted by SR-UE #1, but candecode the SR signal may participate in contention and may be elected asan SR-UE.

FIG. 13 is a diagram referred to for describing in more detail the caseof FIG. 12(c). It is assumed in FIG. 13 that, although SR-UE #2 receivesan SR signal of SR-UE #1 less than a threshold value, SR-UE #2 candecode the SR signal.

In this case, if SR-UE #2 determines to participate in contention inSR-UE determination step 2, SR-UE #2 may be aware of a resource periodreserved by SR-UE #1 by receiving the SR signal transmitted by SR-UE #1in a scanning procedure or a backoff procedure. That is, if the resourceperiod reserved by SR-UE #1 ends, contention (between SR-UE #1 and SR-UE#2) may be started or previous contention may be restarted (or maycontinue). In addition, UEs (i.e., UE #11 and UE #21) that can receiveboth SR signals from SR-UE #1 and SR-UE #2 may be synchronized withSR-UE #1 during the reserved resource period of SR-UE #1 to perform D2Dcommunication and may be synchronized with SR-RE #2 during the reservedresource duration of SR-UE #2 to perform D2D communication.

Furthermore, an entire resource region may be divided into a pluralityof regions and a specific SR signal may be linked to a specific resourceregion.

FIG. 14 is a diagram referred to for describing the case in which aspecific SR signal is linked to a specific resource region according tothe present invention. Referring to FIG. 14, as an example ofclassifying an entire resource region into N regions, a resource unitconsisting of multiple subframes is defined and an entire resource maybe configured in the form of repeating resource units of sequentialindexes (e.g., 1, 2, . . . , N, 1, 2, N). In this case, the SR signalmay include information about a resource region linked thereto.

More specifically, an operation of the present invention to which theabove-described resource region division scheme is applied is described.That is, when a UE determines whether to participate in contention to beelected as an SR-UE, the UE considers the strength of a received SRsignal. If resource regions linked to the SR signal are distinguishableresource regions, the case may occur in which the strength of the SRsignal exceeds a threshold value in a specific resource region and thestrength of the SR signal does not exceed the threshold value in anotherresource region.

In this case, whether to participate in contention to be elected as anSR-UE may be determined according to the following criteria.

-   -   Contention participation criterion 1: If there is any one signal        exceeding the threshold value, a UE is synchronized with an        SR-UE that transmits the corresponding signal. That is, the UE        performs SR-UE determination step 3.    -   Contention participation criterion 2: Even when a signal        exceeding a threshold value is present, no signal exceeding the        threshold value is found in a region except for a resource        region linked to the corresponding signal, a UE participates in        contention. That is, SR-UE determination step 4 described above        is performed.

In this case, the number of UEs becoming SR-UEs decreases whencontention participation criterion 1 is used as compared with the caseof using contention participation criterion 2. That is, a probability ofbeing elected as the SR-UE decreases.

Contention for being elected as the SR-UE in SR-UE determination step 4may be performed according to the following schemes.

-   -   SR-UE contention scheme 1: A scheme of performing random backoff        without determining a resource region to be used.

In SR-UE determination step 4, a contention procedure is performed as inthe case in which a resource region is not divided and a UE that winscontention may transmit an SR signal starting from a backoff completiontime. In other words, a resource region to be reserved at a backoffcompletion time is determined. This resource region may be reserved byasynchronously configuring a resource region non-overlapping a resourceregion of another SR-UE as illustrated in FIG. 14(a) or may besynchronously configured as illustrated in FIG. 14(b).

When a synchronous scheme is used, the SR signal should includeinformation about a boundary so as to recognize the boundary of dividedresource regions. (e.g., a value as to after how many symbols or afterhow many subframes the boundary of divided resource regions is located).

-   -   SR-UE contention scheme 2: A scheme of performing random backoff        after determining a resource region to be used.

For divided resource regions, some of resource regions unoccupied byanother SR-UE are selected to be used and backoff is performed withrespect only to corresponding resource regions.

FIG. 15 is a diagram referred to for describing SR-UE contention scheme2.

Referring to FIG. 15, it can be appreciated that, unlike SR-UEcontention scheme 1, if SR-UE #2 selects resource region 3 from amongthree divided regions, a backoff procedure is not performed with respectto resource region 2 in SR-UE contention scheme 2.

Although a maximum backoff value that can be selected when randombackoff is performed in SR-UE determination step 4, i.e., a backoffwindow, is set to the same value, the effect of random backoff may varyaccording to which contention scheme (e.g., SR-UE contention scheme 1 orSR-UE contention scheme 2 described above) is used.

In other words, as an interval capable of completing backoff increases,an effect of selecting a random value from a small backoff window mayarise. For example, in FIG. 15, decreasing backoff only in resourceregion 3 by determining that only resource region 3 can be used may beregarded as configuring a very large backoff value relative todecreasing a backoff value in both resource region 2 and resource region3 by determining that both resource region 2 and resource region 3 canbe used. To solve such inequality, a valid backoff window may bedefined. The valid backoff window may be defined as normalizing acommonly applied backoff window value to the amount of usable resourceregions.

In this case, as described in FIG. 13, UEs (e.g., UE#11 and UE#21) thatcan receive signals from SR-UE #1 and SR-UE #2 may be synchronized withSR-UE #1 in a resource reservation period of SR-UE #1 to perform D2Dcommunication and may be synchronized with SR-UE#2 in a resourcereservation period of SR-UE#2 to perform D2D communication.

Meanwhile, since an SR-UE that wins contention reserves and uses aresource, if resource reservation is statically performed, it isdifficult to factor in UE mobility and a time-varying networkcharacteristic and an inequality problem in which only a specific UEcontinues to operate as an SR-UE may occur. Accordingly, to solve theabove problems, a valid time period capable of operating as the SR-UEmay be preset or the valid time period may be included in an SR signal.Upon receiving the SR signal including the valid time interval, UEs maybe aware of a remaining valid time and, if the valid time expires, theUEs may resume performing random backoff starting from an end time ofthe valid time regardless of whether the UEs operate as SR-UEs. Inaddition, an SR-UE (e.g., SR-UE #1) that has acquired a valid time fromanother SR-UE (e.g., SR-UE #2) may transmit an SR signal including aremaining valid time of the other SR-UE (i.e., SR-UE #2) except for analready consumed valid time thereof (i.e., SR-UE #1) so as to apply thesame valid time. Using this method, if a predetermined time periodelapses, all UEs or SR-UEs that are synchronized based on a specificSR-UE may simultaneously perform contention again in the same condition.

That is, in SR-UE determination step 1, if no SR signal greater than athreshold value is received or a reserved valid time period expires, aUE may participate in contention for electing an SR-UE and performs arandom backoff procedure as in SR-UE determination step 4. The randombackoff procedure may be continuously performed while deferring backoffin a corresponding reservation duration transmitted from another SR-UEor the random backoff procedure may be performed in a correspondingresource region by selecting an available resource region afteracquiring synchronization from the corresponding SR-UE.

Further, upon receiving an SR signal from an SR-UE, a UE may relay thereceived SR signal in the same SR transmission period as that of theSR-UE or relay the received SR signal in an SR transmission perioddifferent from that of the SR-UE. For convenience of description, a UEthat receives an SR signal from an SR-UE and then relays the received SRsignal will be referred to as an SR-relay UE. However, the SR-relay UEshould not be interpreted as a UE for simply relaying the SR signal and,in some cases, the SR-relay UE should be interpreted as a UE that canrelay a signal other than the SR signal.

FIGS. 16 and 17 illustrate the case in which an SR-relay UE (e.g., UE#11) that has received an SR signal of an SR-UE (e.g., SR-UE #1) relaysthe SR signal.

In FIG. 16, it is assumed that UE #2 can operate as an SR-UE. As opposedto FIG. 13, even when SR-UE #2 is out of coverage of SR-UE #1, if SR-UE#2 can receive resource reservation information of SR-UE #1 from UE #11corresponding to an SR-relay UE, SR-UE #2 may form a cluster thereof(i.e., UE #12) by reserving a resource region while excluding acorresponding resource.

In this case, whether a UE becomes a new SR-UE is determined by the sizeof a signal directly detected from another SR-UE. In the case in whichan adjacent UE relays information of another SR-UE, even when a signalof large size is received from the adjacent UE, if a level of a signaldirectly received from the SR-UE is low, the UE may still performcontention for becoming the SR-UE.

Accordingly, as illustrated in FIG. 17, even when UE #11 operates as anSR-relay UE and an SR signal that SR-UE #2 receives through the SR-relayUE (i.e., UE #11) is greater than a threshold value, if a level of asignal received from SR-UE #1 does not exceed the threshold value, SR-UE#2 may participate in contention.

SR Relay and Frame Structure

Hereinafter, the structure of an SR signal transmitted by an SR-relay UEaccording to the present invention will be described.

A transmission resource of an SR signal that an SR-relay UE relays maybe defined in many ways. For example, a UE that receives an SR signaltransmitted by an SR-UE may transmit the same signal in the sameresource region as a resource region used by the SR-UE starting from thenext SR transmission period after an SR transmission period elapses.

As another example, an SR signal transmission period for the SR-relay UEmay be defined. In this case, the SR signal relayed by the SR-relay UEmay not be the same as the SR signal of the SR-UE. The SR signaltransmission period of the SR-relay UE may be (consecutively) allocatedafter an SR signal transmission period of the SR-UE or may be allocatedwithin the SR signal transmission period of the SR-UE once or repeatedly(specific numbers of times) with a specific interval. The specificinterval may be predetermined or may be included in a higher layersignal (e.g., RRC) or in a signal transmitted by the SR-UE.

FIG. 18 illustrates the case in which an SR signal transmission resourceregion of an SR-relay UE and an SR signal transmission resource regionof an SR-UE are consecutively allocated. In FIG. 18, since the SR-relayUE should transmit an SR signal in the next resource region (e.g.,subframe) after receiving the SR signal from the SR-UE, a transmission(Tx)-reception (Rx) switching period (e.g., a guard time) needs to bedefined between the SR signal transmission region of the SR-UE and theSR signal transmission region of the SR-relay UE.

FIG. 19 illustrates the case in which an SR signal transmission resourceregion of an SR-relay UE is allocated after a specific interval, i.e.,an SR relay interval, from an SR signal transmission resource region ofan SR-UE. Although, in FIG. 19, the SR transmission resource region ofthe SR-relay UE is allocated once within one SR transmission period, thepresent invention is not limited to the case shown in FIG. 19 and itshould be interpreted that the present invention may be applied evenwhen a plurality of resource regions having a plurality of intervals isallocated.

Especially, when an interval consists of multiple subframe units, aninterval value/size may vary according to FDD and TDD. For example,while an interval in FDD may be provided as subframes corresponding to amultiple of 4 or 8 and an interval in TDD may be provided as subframescorresponding to a multiple of 5 or 10. Therefore, when an interval is 8subframes in FDD, if the SR signal transmission subframe of the SR-UE is#n, the SR signal transmission resource region of the SR-relay UE may beallocated in #n+8 (or #n+8, #n+16, . . . etc.).

In addition, one or more relays may be configured to relay an SR signalusing resource regions divided in a frequency region as well as in atime region. Especially, the frequency region in which the SR signal isrelayed may not be equal to an SR signal transmission frequency regionof the SR-UE. For example, when the SR-UE uses center 6 RBs, theSR-relay UE may use frequency regions other than the center 6 RBs anddifferent SR-relay UEs may use different frequency regions.

The SR-UE may repeatedly reserve a specific resource region (i.e., aspecific repeated resource region) to be used for D2D communication in acluster thereof and information about the specific resource region maybe broadcast by being included in the SR signal.

FIG. 20 illustrates the case in which specific repeated resource regionsare reserved by an SR-UE and allocated to D2D communication. SR-UE #2that has received information about corresponding resource regions froman SR-relay UE of SR-UE #1, i.e., from SR-relay UE #11, may selectresource regions to be used thereby (i.e., SR-UE #2) excluding resourceregions reserved by SR-UE #1. In this case, a D2DSS may indicate an SRsignal or include the SR signal.

Information about the reserved resource region may include atransmission interval from an SR signal transmission timing and atransmission period and include a count indicating how many times theresource region is repeated and a transmission end timing. Resourcereservation information transmitted by a relay may be equal to resourcereservation information included in a D2DSS transmitted by an SR-UE whena timing at which an SR-relay UE transmits the D2DSS is predetermined,i.e., when a reception UE is aware of a corresponding D2D transmissiontiming. If the reception UE is not aware of the D2D transmission timing(and/or if a timing at which the SR-relay UE transmits the D2DSS is notpredetermined), the resource reservation information transmitted by therelay may be (re)configured according to the D2DSS timing at which theSR-relay UE transmits the D2DSS.

Meanwhile, when a cluster formed by the SR-UE corresponds to partialcoverage, the information about a corresponding reserved resource regionmay include information about a resource region explicitly used in WANcommunication or a resource region excluding the resource region used inWAN communication may be reserved. In addition, a resource region inwhich the SR-relay UE relays the SR signal may be identically reserved.

D2D data communication using a resource reserved by each cluster may beperformed using a scheme in which an SR-UE schedules transmission andreception resource regions of UEs or a distributed scheme only on thereserved resource. When the distributed scheme is used, for example, ifreservation information is transmitted in the D2DSS, a specific part ofa first appearing resource region (e.g., a first appearing subframeetc.) may be used as a D2D communication contention duration and anSR-UE elected in the contention duration may perform D2D transmission orreception using a remaining reservation resource.

Meanwhile, a transmission period of the D2DSS may be changed using ascheme of repeatedly transmitting some or all of the D2DSS oradditionally inserting a garbage signal (a dummy signal that does notinclude information). The reason is to solve a problem in the case inwhich, when a contention-based resource is initially reserved, a randombackoff completion time of an SR-UE is not equal to the boundary ofresource region periods divided by the SR-UE that has first reserved theresource.

FIG. 21 illustrates the case in which a random backoff completion timeof a D2DSS is not equal to the boundary of resource region periodsdivided by an SR-UE. In FIG. 21(a), backoff is ended in the middle of asubframe and SR-UE #2 may attempt to transmit the D2DSS. As illustratedin FIG. 14, although such a resource reservation scheme may be oneembodiment that is operable in the present invention, a resource regionboundary of SR-UE #2 becomes different from that of SR-UE #1, therebycausing a problem such as inefficient resource use. Therefore, whenbackoff is ended in the middle of a subframe, the D2DSS may berepeatedly transmitted until the start timing of the next subframe toconsecutively occupy a resource, as illustrated in FIG. 21(b). Then, asubframe boundary can be equalized with a resource boundary of SR-UE #1.That is, upon transmitting the D2DSS, if a backoff completion timing isnot identical to a subframe boundary, the SR-UE may repeatedly transmitthe D2DSS so as to continuously occupy the resource until the starttiming of the next subframe boundary, thereby preoccupying a resourceregion in a subframe equalized state.

Accordingly, a threshold/configuration/regulation indicating that theSR-UE should transmit the D2DSS a predetermined number of times or more,that is, indicating a minimum transmission number of times, may bepresent. Further transmission exceeding the minimum transmission numberof times may be optionally configured according to a given situationsuch as a radio communication environment. For example, if the backoffcompletion timing is located in the middle of a subframe, additionaltransmission of the D2DSS exceeding the minimum transmission number oftimes may be performed and the additionally transmitted number of timesmay be flexibly configured up to the boundary timing of the nextsubframe. However, if the backoff completion timing is not located inthe middle of a subframe, the D2DSS may be configured to be transmittedonly the minimum transmission number of times.

FIG. 22 is a diagram referred to for indicating a detailed example of aD2DSS. In FIG. 22, it is assumed that a D2DSS of an SR-relay UE isconsecutively transmitted after a D2DSS signal of an SR-UE istransmitted. It is assumed that a D2DSS is configured such that each ofa PSS and an SSS is repeated four times and then a physical D2Dsynchronization channel (PD2DSCH) appears so that the D2DSS isconfigured by PSS (4+α symbols)+SSS (4 symbols)+PD2DSCH (6 symbols). AUE ID etc. may be obtained from the PSS and the SSS and resourcereservation information may be transmitted on a PD2DSCH.

In FIG. 22, an example of variably configuring the number of times ofPSS repetitions is shown in order to variably define a D2DSStransmission period. That is, an SR-UE should repeatedly transmit thePSS at least four times and, only when random backoff ends in the middleof a subframe, the SR-UE may optionally increase the number of times ofPSS transmission by a (where a is defined as one of 0 to 11 or 13).

Further, various modifications may be made. The PSS and SSS may bedistinguished and an SS specific to such usage may be separately definedso that the SS can be repeatedly transmitted a predetermined number oftimes before or after PSS transmission.

Multi-Hop Relaying of SR

Even when a UE does not directly receive a D2DSS/PD2DSCH from an SR-UE(i.e., an SR signal received from the SR-UE is less than a thresholdvalue), if the UE receives the D2DSS/PD2DSCH from an SR-relay UE (i.e.,an SR signal received from the relay UE is greater than the thresholdvalue), the UE may operate as the SR-relay UE similarly to theabove-described relaying scheme of an SR according to the presentinvention. That is, when multi-hop relaying is performed according tothe present invention, the D2DSS etc. may be transmitted throughmultiple hops and the size of a cluster synchronized with one SR-UE mayincrease in proportion to the square of the number of hops.

In this case, the number of hops through which the D2DSS etc. is relayedmay be limited to properly regulate the size of a cluster using the sameSS. To this end, the D2DSS/PD2DSCH may include a hop count. That is,when a UE determines whether to relay the D2DSS/PD2DSCH receivedthereby, the UE checks the hop count of the D2DSS/PD2DSCH receivedthereby. If the hop count is greater than a predetermined value (e.g.,N), the UE does not operate as the SR-relay UE and, if the hop count isless than or equal to the predetermined value (i.e., N), the UE operatesas the SR-relay UE. If the UE receives multiple D2DSSs/PD2DSCHs havingdifferent hop counts, the UE may determine whether to relay theD2DSSs/PD2DSCHs based on a minimum hop count or a maximum hop count. Inthis case, upon performing a relaying operation, the UE should set thehop count of a D2DSS/PD2DSCH transmitted thereby to a value obtained byadding 1 to the minimum hop count of a received D2DSS/PD2DSCH.

Meanwhile, whether the UE that has received the D2DSS/PD2DSCH operatesas the SR-UE may be determined similarly to the above case of one-hoprelaying. That is, if the hop count of the D2DSS/PD2DSCH received withthe threshold value or more is greater than a predetermined value (K,where K≦N), the UE may operate as the SR-UE. That is, the UE mayparticipate in contention. The predetermined value (i.e., K) may betransmitted through the D2DSS/PD2DSCH. The UE may receive multipleD2DSSs/PD2DSCHs having different hop counts. In this case, whether theUE operates as the SR-UE is determined based on i) a minimum or maximumhop count or ii) a D2DSS/PD2DSCH having received power of the bestquality. The case in which whether the UE operates as the SR-UE isdetermined based on the quality of received power will now be describedby way of example. If D2DSSs/PD2DSCHs transmitted from the same SR-UEare relayed by different UEs using different resources, one UE mayreceive multiple different D2DSSs/PD2DSCHs having the same hop countand, in this case, whether the UE operates as the SR-UE may be based onthe quality of a signal into which the D2DSSs/PD2DSCHs of the same hopcount originated from the same SR-UE are combined as one receivedsignal.

FIG. 23 is a diagram referred to for describing a multi-hop relay of anSR. In FIG. 23, it is assumed that a threshold value (i.e., N) used todetermine whether to relay a D2DSS/PD2DSCH is set to 3 and a threshold(i.e., K) of a hop count of the D2DSS/PD2DSCH is set to 2. In this case,N and K may be predetermined, may be configured through higher layersignaling (e.g., RRC), or may be included in a physical layer signal(e.g., a D2DSS/PD2DSCH).

In FIG. 23, UE #1 that has received a D2DSS/PD2DSCH from SR-UE #1 relaysthe D2DSS/PD2DSCH including information of “hopcount=2” in a relayD2DSS/PD2DSCH transmission period (herein, this period may be the sameor different from a D2DSS/PD2DSCH transmission duration of the SR-UE).Upon receiving the D2DSS/PD2DSCH, UE #2 relays the D2DSS/PD2DSCHincluding information about “hopcount=3” and, similarly, upon receivingD2DSS/PD2DSCH, UE #3 relays the D2DSS/PD2DSCH including information of“hopcount=4”. However, UE #4 that has received the D2DSS/PD2DSCH relayedfrom UE #3 does not operate as an SR-relay UE because the hop count ofUE #3 is greater than N=3.

Meanwhile, the K value serves to properly maintain the number of SR-UEsand may be different from the value N. If K and N are different, the UEmay operate as an SR-UE even upon receiving the D2DSS/PD2DSCHcorresponding to hopcount=K+1, . . . , N. Accordingly, in FIG. 23, UE #3as well as UE #4 may operate as an the SR-UE.

Especially, coverage is important in an SR signal such as theD2DSS/PD2DSCH and an upper limit of power of the SR signal may be higherthan a normal communication signal. That is, the upper limit of power ofthe communication signal may be adjusted by an eNB through an SR-relayUE in a partial coverage case. Even in this case, the SR signal such asthe D2DSS/PD2DSCH (e.g., direct transmission of a cluster head orrelaying transmission of UEs) may be transmitted at higher transmissionpower than transmission power of the communication signal. For example,while the upper limit value of power of the communication signal istransmitted through the SR signal such as the D2DSS/PD2DSCH, the SRsignal may use a larger value than the upper limit value of thecommunication signal. For example, the transmission power value of theSR signal may use the maximum transmission power of a UE, a new upperlimit value obtained by adding a predetermined offset to the upper limitof power of the communication signal, a specific value obtained throughthe D2DSS/PD2DSCH, or a preconfigured value.

FIGS. 24 and 25 are diagrams referred to for describing a resourceregion for relay signal transmission.

A D2DSS/PD2DSCH transmission resource of an SR-relay UE may betransmitted using divided resource regions as described in FIGS. 18 to23. FIG. 24 illustrates the case in which, after an SR-UE transmits anSR signal such as D2DSS/PD2DSCH, relay signal transmission periods offirst, . . . N-th hop are configured. A predetermined gap may be presentbetween relay periods in consideration of Tx-Rx switching.

As illustrated in FIG. 25, in order to enable a relaying operation of Nhops, N divided D2DSS/PD2DSCH transmission periods may be configured andrelaying may be sequentially performed with respect to a part of theD2DSS and to the PD2DSCH. In this case, a guard period is needed forTx-Rx switching between a part of the D2DSS and the PD2DSCH transmittedwith different hops. The relaying scheme as illustrated in FIG. 25 maybe effective in solving the hidden node problem.

SR-UE Selection

Hereinabove, a method for determining whether a UE operates as anindependent SR-UE or an SR-relay UE, based on the quality(signal-to-interference-plus-noise ratio (SINR), reference signalreceived power (RSRP), etc.) of the received D2DSS/PD2DSCH or on a hopcount, has been described.

Hereinafter, a method for determining with which SR-UE the UE thatreceived distinguishable D2DSSs/PD2DSCHs from a plurality of SR-UEs issynchronized will be described.

In this case, a plurality of SR-UEs may include both an SR-UEcorresponding to a synchronization source and an SR-relay UE (forrelaying an SR). Selection of an SR-UE for synchronization may beinterpreted as selection of a D2DSS/PD2DSCH for synchronization andincludes the case in which two or more SR-UEs transmit the same SS inthe same resource region.

FIG. 26 illustrates the case in which an arbitrary UE receivesdistinguishable D2DSSs/PD2DSCHs from a plurality of SR-UEs. For example,UE #0 may (simultaneously) receive D2DSSs/PD2DSCHs from SR-UE #0 andSR-UE #1. In this case, when the UE selects an SR-UE to be synchronizedtherewith based on the quality of the received SS as described above,the following operations may be performed.

-   -   SR-UE selection scheme #1: a) The case in which one or more        SR-UEs have the quality of a received SS greater than a specific        level γ. a-1) Upon receiving an SS from one SR-UE, the UE is        synchronized with an SR-UE that has transmitted the SS. a-2)        Upon receiving SSs from two or more SR-UEs, the UE is        synchronized with an SR-UE having the lowest hop count. If two        or more SR-UEs have the lowest hop count, the UE may randomly        select an SR-UE and is synchronized with the selected SR-UE.        However, b) if the quality of all received SSs is less than the        specific level γ, the UE may operate as an independent        synchronization source.    -   SR-UE selection scheme #2: Although SR-UE selection scheme 1 is        used, the UE may operate to be synchronized with an SR-UE having        the best quality among received SSs even when the quality of all        received SSs is less than the specific level γ, as opposed to        the SR-UE selection scheme 1.    -   SR-UE selection scheme #3: When an SR-UE having signal quality        higher than a first quality level γ or more among received SSs        is not present, the above scheme may be used to select an SR-UE        with which a UE is to be synchronized by applying a second        signal quality level. That is, signal quality reference values        of multiple levels may be defined. If no SR-UE satisfies a first        reference value, an SR-UE satisfying the reference value of the        next level is selected. For example, reference values of N        levels may be defined as γ(0)>γ(1)> . . . >γ(N−1).

FIG. 27 is a flowchart illustrating an SR-UE selection scheme when alevel value (i.e., N) of the above-described signal quality is applied.As described in SR-UE selection scheme 1, if two or more SR-UEssatisfying a specific quality criterion of a received signal arepresent, an SR-UE having the lowest hop count may be selected and, iftwo or more SR-UEs have the lowest hop count, one of these SR-UEs may berandomly selected.

If no SR-UEs satisfy the reception signal quality of the lowest level inFIG. 27, synchronization is performed with respect to an SR-UE havingthe best reception signal quality among the SR-UEs as in theabove-described SR-UE selection scheme 2.

FIG. 28 is a flowchart illustrating a scheme in which a UE operates asan SR-relay UE or an SR-UE. For a UE to operate as an SR-relay UE or anSR-UE, when a maximum hop count is restricted, it is necessary todetermine whether a maximum hop count of a UE is exceeded as illustratedin FIG. 28. Especially, the scheme illustrated in FIG. 28 may be used toprevent the UE from being synchronized with an SR-UE having a relativelyvery large hop count in spite of good quality of a received SS.

FIG. 29 is a diagram referred to for describing a scheme of applying adifferential signal quality criterion according to a synchronizationpurpose of a UE. As illustrated in FIG. 29, SINR reference values ofmultiple levels (i.e., γ(0), γ(1), and γ(2)) are set and each UE may i)determine a synchronization purpose thereof according to SR-UE selectionscheme 2 described above in relation to FIG. 27 or ii), conversely,apply different reference values according to synchronization purposethereof. For example, the UE may operate as an independentsynchronization source upon detecting no SR-UEs having a receptionsignal quality of γ(0) or more and operate as an SR-relay UE upondetecting an SR-UE having a reception signal quality of γ(1) or more.Conversely, if a specific UE is determined or configured to be a non-SRUE (i.e., a UE that cannot operate a synchronization source although theUE is synchronized with an SR-relay UE or an SR-UE), an SR-UE to besynchronized is selected based on the reception signal quality of γ(0)and a UE determined or configured to operate as an SR-UE selects anSR-UE to be synchronized based on the reception signal quality of γ(1).

-   -   SR-UE selection scheme #4: This scheme causes a UE that has        received distinguishable D2DSSs/PD2DSCHs from one or more SR-UEs        to select an SR-UE to be synchronized. A reception signal        quality reference value may be secondarily applied to SR-UE sets        divided based on a hop count and multiple quality levels may be        applied even to a hop count as described in SR-UE selection        scheme 2. In SR-UE selection scheme 4, an SR-UE different from        an SR-UE selected in SR-UE selection schemes 1 to 3 may be        selected according to a reference value of a hop count. For        example, when an SINR of SR-UE #2 of hop count 2 is higher than        an SINR of SR-UE #0 of hop count 1 and the both SR-UE #2 and        SR-UE #0 satisfy an SINR reference value, if a reference hop        count is set to 2, SR-UE #2 is selected when SR-UE selection        scheme 44 is applied, whereas SR-UE #1 is selected when SR-UE        selection schemes 1 to 3 are applied. Obviously, if the        reference hop count is set to 1, the same SR-UE may be selected        in SR-UE selection schemes 1 to 3 and in SR-UE selection scheme        4.

Furthermore, after SR-UE selection scheme 4 is applied to a maximumavailable hop count, at least one of SR-UE selection schemes 1 to 3 maybe applied. In this case, SR-UEs not exceeding the maximum hop count areselected first and an SR-UE is selected by comparing reception signalquality of the respective SR-UEs.

SR-UE (Re)Selection and Relay Capability Indication

A description is now given of the case in which all SR-UEs detected by aUE have a maximum available hop count in indicating SR-UE (re)selectionand relay capability.

In relation to SR-UE selection, SR-UE selection schemes 1 to 4 may beapplied to SR-UEs having a maximum hop count. In this case,synchronization with an SR-UE means that a UE can operate as a non-SR UEor a dependent SR-UE and cannot operate as an SR-relay UE.

When there are no SR-UEs having reception signal quality of a specificlevel or more, a UE operation may be differently defined according towhich of SR-UE selection schemes 1 to 4 is applied. For example, whenSR-UE selection scheme 2 is used, a UE may be synchronized with an SR-UEhaving the best signal quality among SR-UEs so that the UE operates as anon-SR-UE or a dependent SR-UE. When SR-UE selection scheme 1 is used,the UE may operate as an independent SR-UE so that the UE selects anduses a resource independently of other SR-UEs. If SR-UE selection scheme3 is used, the UE may be synchronized with an SR-UE having the bestsignal quality among SR-UEs having signal quality within a predeterminedrange (i.e., below a first level and above a second level, where firstlevel>second level) to operate as a non-SR-UE or a dependent SR-UE andthe UE operates as an independent SR-UE when the SR-UEs have signalquality less than a predetermined range (i.e., below the second level).

Meanwhile, UEs (including a non-SR-UE, an SR-relay UE, and a dependentSR-UE) synchronized with a specific SR-UE may perform a reselectionprocedure. The reselection procedure in the present invention indicatesthat an SR-UE selection procedure using SR selection scheme 1 to SR-UEselection scheme 4 described above is performed again.

A method for performing the reselection procedure in every valid timeperiod of an SR-UE will now be described first. As described earlier, anoperation as an SR-UE in an SR-UE election procedure may be valid onlyduring a finite specific time period and valid time information may beincluded in a transmitted D2DSS, PD2DSCH, etc. Therefore, upon receivingthe information, UEs simultaneously re-performs a synchronizationselection procedure when a valid time ends so that the UEs aresynchronized with another SR-UE or are elected as an SR-UE. To detect anSR-UE that does not belong to an existing cluster, a scanning timeperiod should be further present after the valid time expires. Upondetecting an SR-UE during the scanning time period, the UEs may besynchronized with the SR-UE. In contrast, when no SR-UE is detected inthe scanning time period, the UEs may perform a random backoff procedureso that the UEs become the SR-UE or are synchronized with a newlyelected SR-UE (that may be same as an SR-UE of an existing cluster)among members of the existing cluster. An additional time period may beperiodically defined so as to perform SR-UE reselection separately fromthe valid time period.

In addition, a UE may detect a prioritized SR-UE or sense variation ofan SR-UE with which the UE is synchronized i) at a timing at which SRtransmission of an SR-UE thereof and an adjacent SR-UE is expected orii) at an arbitrary timing while the UE operates in a reception mode. Inthis case, the UE may perform a reselection procedure and the UE may beimmediately synchronized with a reselected SR-UE or may be synchronizedwith the reselected SR-UE at a (periodically) predetermined timing. Inthis case, the UE may be synchronized with a new SR-UE even when a validtime period of a currently synchronized SR is not ended.

The valid time information transmitted by an SR-UE corresponds toinformation about lifetime as a synchronization source of the SR-UE andmay occupy many bits because the information is about time. Meanwhile,since the synchronized UE only needs to acquire information about whenthe valid period expires, accurate time information is needed onlyduring a partial time duration immediately prior to expiry of the validtime and, during the other time durations, only an index indicating thatan expiry time is sufficient may be transmitted to the synchronized UE.For example, if the valid time period information can be represented as0 to N, N indicates that the time period is not ended prior to N periodsrather than indicating that the time period is ended after N periods (orabsolute time N). 0 may be used instead of N. Therefore, the SR-UE maytransmit N (or 0) for a while based on an accurate valid time knownthereto and, if an actual valid time is shorter than N (or 0), the SR-UEmay transmit a value such as N−1, N−2, . . .

Upon selecting an SR-UE, a UE may also consider the valid timeinformation. That is, the UE may pre-exclude an SR-UE having a validtime less than a specific value K from the SR-UE selection procedure or,when two or more SR-UEs having the same hop count and the same receptionsignal quality are detected in the SR-UE selection procedure, the validtime information may be used as a criterion for selecting one of theSR-UEs.

Meanwhile, in selecting the SR-UE, it may be important whether the SR-UEcan support a relay function and a UE that desires to access a networkwill try to be synchronized with the SR-UE supporting the relayfunction. Accordingly, the SR-UE may include an index indicating whetherthe SR-UE has relay capability in a transmitted D2DSS, PD2DSCH, etc. Forexample, a relay capability field may be set indicating ‘1’ for a relaycapable function and ‘0’ for a relay incapable function. Therefore, theUE that desires to access the network selects SR-UEs supporting relayfrom among detected SR-UEs and then selects an SR-UE with which the UEis to be synchronized by applying one of the above-described SR-UEselection schemes (i.e., SR-UE selection schemes 1 to 4).

FIG. 30 is a diagram referred to for describing the case in which aD2DSS/PD2DSCH is sequentially relayed when a maximum hop count is set.As illustrated in FIG. 30, when a maximum available hop count is N, aD2DSS/PD2DSCH may be sequentially relayed from UE1 within a network toadjacent UE2, UE3, . . . . Assuming that transmission of UE1 correspondsto hop count 1, whether an SR-UE supports relay may be determined byjudging whether the SR-UE is capable of transmitting D2D data to thenetwork or receiving the data from the network.

When a UE having a hop count n (where N≧n>1) sets relay capability to‘1’ (i.e., relay capable), a UE receiving a signal including this relaycapability indication may interpret capability of the UE as follows.

Relay determination 1) UE n supports data relay to a network. That is,UE n is capable of supporting ‘UE-UE data relay’ and UE k also support‘UE-UE data relay’ for all k (where 1<K<n). UE 1 also supports relay.

Relay determination 2) UE n does not support data relay to a network.However, UE 1 supports relay.

When a UE having a hop count n (where N≧n>1) sets relay capability ‘0’,a UE receiving a signal including this relay capability indication mayinterpret capability of the UE as follows.

Relay determination 3) UE n does not support data relay to a network. Inaddition, UE 1 does not support relay.

Meanwhile, when a UE having hop count n (where N≧n>1) corresponds torelay determination 2) (i.e., UE n does not support relay), the UE mayset relay capability to ‘0’. In this case, an operation of UE n, (whereN≧n>1) is as follows.

UE n selects an SR-UE having the best signal quality among receivedD2DSSs/PD2DSCHs of an SR-UE using at least one of SR-UE selectionschemes 1 to 4 described above and adds 1 to a hop count of the selectedD2DSS/PD2DSCH to set the added hop count as a hop count thereof. If arelay capability field of the selected D2DSS/PD2DSCH is set to ‘0’, theUE should also set a relay capability field thereof to ‘0’. If the relaycapability field of the selected D2DSS/PD2DSCH is set to ‘1’, UE n setsthe relay capability field to ‘1’ when UE n supports ‘UE-UE data relay’and to ‘0’ when UE n does not support ‘UE-UE data relay’.

Upon selecting the best SR-UE, a UE may also consider a value of therelay capability field. For example, highest priority may be assigned toan SR-UE supporting relay to select only the SR-UE supporting relayregardless of a hop count or reception signal quality and then considera minimum hop count, maximum reception signal quality, etc. If priorityis assigned by different schemes to selection reference elements such asa hop count, signal quality, relay support/relay non-support, and validtime information, various SR-UE selection schemes may be derivedsimilarly to SR-UE selection schemes 1 to 4.

Particularly, different priorities may be applied according to a D2D useenvironment. For example, in a normal situation, the highest priority isassigned to a hop count to select an SR-UE and, only when connection toan adjacent network is urgent in an emergency situation, the highestpriority is assigned to relay support/relay non-support to configure D2Dcommunication synchronized with an adjacent network up to a maximum hop.

FIG. 31 is a diagram referred to for describing the case in which aPD2DSCH becomes a single frequency network (SFN). As illustrated in FIG.31, when a PD2DSCH becomes an SFN, there are UEs supporting relay andUEs not supporting relay. That is, a plurality of UEs having hop count nmay be present and the respective UEs may have different relay supportfunctions. For example, in FIG. 31, although UE2 and UE3 have the samehop count of 1, UE2 may support relay and UE3 may not support relay.Accordingly, in this case, since a UE is unaware of capability ofdifferent UEs having the same hop, the relay capability field may be setto ‘0’ irrespective of an SR-UE selected thereby. Alternatively, therelay capability field may be set to ‘1’ and this case may beinterpreted as the case of relay determination 2) described above. Thatis, although information as to whether data relay to a network issupported cannot be transmitted, since information indicating that a UEhaving a hop count of 1 is located within the network is identicallyapplied to all UEs synchronized with UE 1, the relay capability fieldmay be used to transmit such information. If a UE that has received aD2DSS/PD2DSCH having a hop count of n (where N≧n>1) receives the relaycapability field of 1, this should be interpreted as indicating that aUE having a hop count of 1 is located in the network.

Meanwhile, since an indicator indicating whether to support relay isrelated to UE capability, the indicator may be designed to betransmitted in the form of a signal such as a discovery signal ratherthan to be included in the D2DSS/PD2DSCH. In this case, a UE thatdesires to access the network through a D2D link, i.e., a UE thatexpects data relay, is synchronized with a D2DSS/PD2DSCH selected usingSR-UE selection schemes 1 to 4 and performs a procedure of discovering aUE supporting a data relay function among UEs transmitting theD2DSS/PD2DSCH. If no UEs supporting the data relay function arediscovered, the UE performs a D2DSS/PD2DSCH selection procedure againusing SR-UE selection schemes 1 to 4.

In the D2DSS/PD2DSCH reselection procedure, a previous D2DSS/PD2DSCHthat fails to discover an SR-relay UE should be excluded. Accordingly, aUE expecting data relay may pre-exclude a D2DSS/PD2DSCH that fails todiscover an SR-relay UE, maximally set a hop count of the D2DSS/PD2DSCHthat fails to discover the SR-relay UE, or minimally set received signalquality, upon performing a synchronization reselection procedure, sothat, the UE may set the corresponding D2DSS/PD2DSCH to have the lowestpriority upon applying the SR-UE selection scheme.

Regardless of which SR-UE selection scheme is used, the reselectionprocedure is useful to discern related information in a synchronizationprocedure because the synchronization procedure should be sequentiallyrepeated according to a criterion of a corresponding SR-UE selectionscheme when a UE fails to discover an SR-relay UE in a selectedD2DSS/PD2DSCH.

Accordingly, in order to indicate information representing that there isa UE supporting data relay among UEs transmitting the D2DSS/PD2DSCH, therelay capability field may be defined in the D2DSS/PD2DSCH. For example,an eNB may command one or more UEs located within a network to transmitthe D2DSS/PD2DSCH by an SFN scheme, wherein the eNB may command aspecific UE set including a UE supporting data relay to transmit theD2DSS/PD2DSCH and set the relay capability field to 1.

FIG. 32 is a diagram referred to for describing the case in which UEssupporting relay and UEs not supporting relay are mixed. In FIG. 32, itis assumed that UE2 supports relay and UE1, UE3, and UE4 do not supportrelay. In this case, the eNB may command UEs to differently set a relaycapability field upon commanding UE1 and UE2 to transmit D2DSSs/PD2DSCHsand commanding UE3 and UE4 to transmit D2DSSs/PD2DSCHs. When the eNBcommands UE1 and UE2 (or UE3 and UE4) to transmit D2DSSs/PD2DSCHs, UE 5that desires relay connection may be synchronized with D2DSSs/PD2DSCHstransmitted from UE1 and UE2 (or UE3 and UE4) with priority over otherD2DSSs/PD2DSCHs.

Then, when a UE receives D2DSSs/PD2DSCHs, the UE may recognize thatthere is a UE supporting data relay among UEs that transmit theD2DSSs/PD2DSCHs, if the corresponding signals are transmitted by UE(s)within a network, a hop count value is 1, and a relay capability fieldis set to ‘1’. A UE that desires data relay is first synchronized with aD2DSS/PD2DSCH having the relay capability field set to 1. The UE maythen discover a UE supporting relay through an additional procedure,configure a D2D link with respect to a corresponding UE to connect tothe D2D link, and transmit data using a resource linked to theD2DSS/PD2DSCH by a broadcasting scheme according to traffic attributes.

Relay Triggering

FIG. 33 is a diagram referred to for describing relay triggering when noSR-relay UEs are present. As described above, a UE may operate as anSR-relay UE when the strength of a D2DSS received from an SR-UE iswithin a predetermined range. That is, in order for the UE to operate asthe SR-relay UE, the UE should decode corresponding information by beingsynchronized with a D2DSS of the SR-UE. Therefore, while a signalreceived from the SR-UE is higher than a specific level, the UE may bedefined as operating as a non-SR-UE when the received signal strength istoo high.

However, a situation in which no SR-relay UE is present may occur asillustrated in FIG. 33. In this case, if an adjacent SR-UE starts toperform D2D transmission in an unsynchronized state, unexpectedinterference may occur. Accordingly, even when strength of a signalreceived from an SR-UE is excessively high and a UE does not satisfy athreshold value for operating as an SR-relay UE, if the eNB or the SR-UEcommands the UE to operate as the SR-relay UE, the UE may besynchronized with the eNB or the SR-UE so as to operate as the SR-relayUE. An (unsynchronized) SR-UE that has received a D2DSS from theSR-relay UE is synchronized with a relayed D2DSS.

Mobility Support and Hop Count for D2DSS Reselection

If an attribute of a D2DSS with which a UE is synchronized is changed ora D2DSS having a higher priority than the D2DSS with which the UE issynchronized is detected, the UE may reselect the D2DSS. That is, asdescribed above, an operation scheme of the UE may be differentlydefined according to various elements such as signal strength of areceived D2DSS, a hop count, and an indicator indicating whether tosupport relay. For example, if the UE operates to be synchronized with acorresponding D2DSS only when the quality of the received D2DSS is abovea specific level, the UE may determine that the attribute of the D2DSSis changed when the strength of the received D2DSS (a PSS/SSS in thecase of being synchronized with an eNB) is continuously attenuated to avalue less than a threshold value and may perform a reselectionprocedure.

FIG. 34 illustrates the case in which an attribute of a D2DSS with whicha UE is synchronized is changed due to mobility of the UE and a D2DSShaving a higher priority than a D2DSS with which the UE is synchronizedis detected. FIG. 34(A) and FIG. 34(C) illustrate the case in which anattribute of a D2DSS is changed due to mobility of a UE and FIG. 34(B)and FIG. 34(D) illustrate the case in which an attribute of a D2DSS ischanged due to mobility of an SR-UE. However, the UE regards FIGS. 34(A)to 34(D) as the same situation.

In the situation of FIG. 34, a reselection procedure of a UE will now bedescribed.

First, when an attribute of a D2DSS is changed, the UE may attempt toscan an adjacent D2DSS immediately or at a determined timing. The UE mayattempt to scan the D2DSS using information of a D2DSS detected at aprevious scan timing or perform reselection using information of theD2DSS detected at the previous scan timing by omitting a scan trialprocedure and then sequentially attempt to detect a selected D2DSS.

The case in which no D2DSS having priority is detected corresponds tothe case in which there is no D2DSS to replace a current D2DSS, forexample, the strength of a detected D2DSS is lower than a thresholdvalue, a hop count is a maximum value, or a UE is located in an isolatedregion. Accordingly, the UE may operate as an SR-UE by directlytransmitting a D2DSS or maintain a current D2DSS according to whetherattribute change thereof is a level capable of changing an operationmode. For example, if attribute change corresponds to increase of thehop count, the UE operates as the SR-UE when the hop count is a maximumvalue and maintains a current D2DSS when the hop count is not themaximum value.

However, when a D2DSS with priority is detected, the UE may besynchronized with a new D2DSS using one of the above-describedreselection schemes.

In contrast, when attribute change of a D2DSS does not occur, the UE mayscan an adjacent D2DSS at a specific predetermined periodic/aperiodictime/period designated by an eNB/SR-UE/scheduling UE and arbitrarilyselected thereby. In this case, there is no detected D2DSS havingpriority, the UE maintains a current D2DSS and, when there is a detectedD2DSS having priority, the UE may be synchronized to a new D2DSS usingone of the above-described reselection schemes of the present invention.

In the above-described reselection procedure of a UE, when the UEperforms reselection in the case in which a newly detected D2DSS havingpriority is present, a criterion and/or a threshold value of a D2DSShaving priority different from an initially selected D2DSS may beapplied because excessively frequent reselection procedures may causeinstability of a network. To this end, it is desirable to assignpriority (i.e., advantage) to a currently synchronized D2DSS. Forexample, when priority is determined according to the signal quality ofa received D2DSS, fair comparison is performed with respect to alldetected D2DSSs during first selection, whereas, during reselection, aspecific offset is additionally set with respect to the signal qualityof a currently synchronized D2DSS to perform reselection only withrespect to a D2DSS having better signal quality than the quality of acorresponding D2DSS by the offset or more. Similarly, even for a hopcount, the offset may be set with respect to a current D2DSS duringreselection.

Alternatively, the offset may be applied when determination as towhether there is attribute change is made. That is, if no changeexceeding the offset is sensed in a D2DSS of a UE, the UE may notperform the reselection procedure. If such a scheme is used, the case inwhich a D2DSS having priority is detected when attribute change of theD2DSS does not occur is present will not occur.

Especially, when multi-hop relay can be performed, a different offsetvalue per hop may be applied during reselection. Because a D2DSSreselection procedure of a high hop cause much change in terms of anetwork, it is effective to maintain a current state to stabilize thenetwork.

Accordingly, a UE of a low hop may determine that an attribute of aD2DSS has been changed when a UE of a high hop is synchronized with anew D2DSS (when an SFN is used, the same may not be applied) and performa D2DSS scan procedure. The UE of the high hop may reselect a D2DSS andtransmit information about a D2DSS to be pre-changed to the UE of thelower hop before resynchronization, thereby aiding in a scan procedureof the lower hop.

FIG. 35 is a diagram referred to for describing reselection of a D2DSSin consideration of a hop count. FIG. 35(A) illustrates the case inwhich a UE having a hop count of 1 reselects a D2DSS and FIG. 35(B)illustrates the case in which a UE having a hop count of 3 (a maximumhop count) reselects a D2DSS.

As illustrated in FIG. 35, while D2DSS reselection by a UE having a highhop causes subsequent D2DSS reselection, D2DSS of a final hop does notcause change of a network having a high hop. Therefore, according to thepresent invention, a different offset value for each hop may be definedas shown in Table 5. If such a scheme is used, the UE having a high hopcount may limitedly perform reselection due to a high reselectionoffset, whereas a UE having the lowest hop count (or a maximum hopcount) may freely perform reselection due to a low offset. In addition,an offset may be set to ‘0’ with respect to the UE having the lowest hopcount (or maximum hop count) UE so as to exclude difference from firstD2DSS selection only with respect to the UE having the lowest hop count.If a UE can be aware of presence of a UE synchronized therewith, Q(N)may be applied only to the case in which a lower hop of the UE is notpresent.

TABLE 5 Hop Count Offset 1 Q (1) 2 Q (2) (<Q (1)) . . . . . . N (Maximumhop) Q (N) (<Q (N − 1))

D2DSS Transmission Resource Pattern

In the present invention, a transmission resource region of a D2DSS isnot always divided by a relay hop count as illustrated in FIG. 21. Thatis, two or more D2DSS transmission resources may be present within apredetermined D2DSS transmission period and information about such atransmission resource period may be defined to be transmitted in acontrol channel such as a PD2DSCH or transmitted according to apredetermined pattern.

In more detail, a D2DSS may include a transmission-capable resourceregion of N times (where P≧N≧1) during a D2DSS transmission period P andeach transmission-capable timing may have an interval of a K(i) (where1≦i≦N) duration. The UE may select a specific resource region from amongtransmission-capable resource regions of N times to transmit the D2DSS.

FIG. 36 illustrates the case in which P=1000, N=10, and K(i)=1 for allI, in a time unit (subframe) of 1 ms.

Although a specific resource region for D2DSS transmission may beconfigured to have all resource regions or a partial resource regionarbitrarily selected from among all of the resource regions, i) aresource region linked to a hop count may be used or ii) a resourceregion having the lowest signal strength of a received D2DSS may beused.

Meanwhile, one or plural consecutive transmission resources may beconfigured to be allocated at a specific interval rather than aplurality of D2DSS transmission resources being consecutively allocated.According to this scheme, an influence of WAN communication caused byD2DSS transmission can be reduced and reduction of a D2DSS transmissionopportunity can be compensated when consecutive subframes cannot beconfigured as UL as in a TDD system

FIG. 37 is a diagram illustrating the case in which transmissionresources are allocated at a specific interval according to the presentinvention.

Referring to FIG. 37, when an index of a transmission resource for P=1s(i.e., 1000 ms) and N=10 is i, K(i)=5 for all i as illustrated in FIG.37(a), K(i)=1 for odd indexes (i=1, 3, 5, . . . ) as illustrated in FIG.37(b), or K(i)=4 for even indexes (i=2, 4, 8 . . . ) so that a patternconfigured at an interval of 5 ms may be provided between resourceshaving odd indexes. Alternatively, a transmission pattern (10 ms betweenodd indexes) defined to be configured as K(i)=1 for odd indexes (i=1, 3,5, . . . ) and K(i)=9 for even indexes (i=2, 4, 8 . . . ) may beprovided as illustrated in FIG. 37(c).

Particularly, since the TDD system may have the above-described UL-DLconfiguration as shown in Table 2, if K(i) is set to 5 ms to 10 ms, Ntransmission opportunities may be provided within a D2DSS transmissionperiod. For example, if a transmission period is 100 ms and atransmission-capable resource interval is 10 ms, a transmission-capablesubframe of a D2DSS may be determined as SF#100*i+10*j+2 (where i andj=0, 1, 2, . . . ) (SF#2, 12, 22, . . . ). In this case, the UE mayattempt to transmit the D2DSS in one subframe or in two or moresubframes and attempt to detect the D2DSS in the other subframes withrespect to a resource region of 10 uniformly distributed subframeswithin a period of 100 ms (or the UE may attempt to detect the D2DSS inall subframes or the UE may attempt to detect the D2DSS in specific somesubframes and attempt to transmit the D2DSS in some subframes). In thiscase, information about a D2DSS transmission subframe may be transmittedto a UE in coverage of an eNB (an in-coverage UE) through a signal of aneNB physical layer and a signal of a higher layer such as an RRC signalor may be defined to use a prescheduled pattern. In addition, theinformation may be transmitted to a UE out of a coverage area (anout-coverage UE) through a D2D control channel, such as a PD2DSCH, and aD2D data channel.

Particularly, in selecting a subframe in which the UE attempts toperform transmission, the UE may be synchronized with a D2DSS of a highhop count to select a subframe corresponding to a hop count thereof andthen to transmit the D2DSS. However, the UE may be synchronized with aD2DSS having the best reception strength of a D2DSS discovery signal toselect an arbitrary subframe or select a subframe in which the receptionstrength of the D2DSS discovery signal is lowest (i.e., a subframe inwhich other D2DSSs are not detected). This scheme may be identicallyapplied even to an FDD system and, in this case, a D2DSS may beconfigured to be present at a period of 4 ms or 8 ms. In addition, thetransmission pattern is not always defined to have an interval of 5ms/10 ms or 4 ms/8 ms and may have an arbitrary transmission interval.The transmission pattern may be reconfigured to have anothertransmission interval value over time or a specific transmissionresource or all transmission resources may be released.

As illustrated in FIG. 36, a D2DSS transmission resource region mayinclude N D2DSS transmission resources within a predefined or signaledD2DSS transmission period (hereinafter, “P”) (where P≧N≧1). Each of theN D2DSS transmission resources may include an interval of K(i) (where1≦i≦N). In this case, D2D UE#X may i) attempt to transmit a D2DSS in oneresource (e.g., subframe) or in two or more resources and attempt todetect the D2DSS in the other resources, ii) attempt to detect the D2DSSin all resources, or iii) attempt to detect the D2DSS in specificpartial resources and attempt to transmit the D2DSS in some resources,among N D2DSS transmission resources within the D2DSS transmissionperiod P.

In this case, D2D UE#X may mean an SR-UE (a UE providing an SR signal),an independent synchronization source (ISS) UE, or a UE relaying an SRsignal (i.e., an SR-relay UE). For convenience of description, althougha description will be given based on a UE that relays an SR signal, itis apparent that the UE can be extended to other forms or other types ofUEs.

In addition, a resource that a UE or an SR-UE relaying an SR signalselects to attempt to transmit a D2DSS among N D2DSS transmissionresources within a D2DSS transmission period P should be efficientlyselected in consideration of a half duplex (HD) problem (i.e., inabilityto perform a D2DSS reception operation due to a D2DSS transmissionoperation) or an interference problem.

FIG. 38 is a diagram referred to for describing the case in which 3 UEsselect D2DSS transmission resources in a D2DSS transmission resourceregion P, N=4, and K(i)=1 for all i. It is assumed that the UEs areconfigured to select a resource having the lowest detection energy or aresource having energy lower than a predefined (signaled) thresholdvalue.

Under such an assumption, in FIG. 38, when UE1 (in this case, an SR-UE)transmits a D2DSS in the second D2DSS subframe of a K-th period P andUE2 (a UE relaying an SR signal, i.e., an SR-relay UE) that has receivedthe D2DSS transmits a D2DSS using a resource in the third D2DSS subframeof a K′-th period P rather than a resource in the second D2DSS subframein which a D2DSS of high energy is detected, it is undesirable that UE3(a UE relaying an SR signal) transmit the D2DSS using a resource in thesecond D2DSS subframe of a K″-th period P in which low energy isdetected due to a relatively long distance. This is because the D2DSStransmitted through the resource in the second D2DSS subframe of theK″-th period P may generate interference with respect to UEs thatreceive the D2DSS (i.e., the D2DSS transmitted through the resource inthe second D2DSS subframe of the K-th period P) of UE1 (i.e., SR-UE) andUE3 is incapable of receiving the D2DSS transmitted by UE1 (where K, K′,and K″ may be set to have the same value (i.e., the same periodinterval) or different values according to predefined configuration).

Accordingly, in order to solve the above problems, a UE (e.g., a UErelaying an SR signal) or an SR-UE that has detected a D2DSS in an N-thD2DSS transmission resource (e.g., subframe) of a K-th D2DSStransmission period P may be configured to transmit or relay the D2DSSaccording to the following resource selection scheme 1) or resourceselection scheme 2). In this case, such configuration may be limitedlyapplied only to the case in which a D2DSS transmission resource is notselected in linkage with a hop count value.

Resource selection scheme 1: A UE that has detected a D2DSS in an N-thD2DSS transmission resource (e.g., subframe) of a K-th D2DSStransmission period P may be configured to transmit (or relay) the D2DSSin an N′-th D2DSS transmission resource of a K′-th D2DSS transmissionperiod. In this case, K′ may be defined as (K+P_OFFSET) (e.g., P_OFFSETis an integer greater than or equal to 1 (or 0)) and N′ may be definedas ((N+N_OFFSET) mod N) (e.g., N_OFFSET is an integer greater than orequal to 1 (or 0), where operation of (A mod B) denotes a remainderobtained by dividing A by B). FIG. 39 illustrates the case in which allof P_OFFSET and N_OFFSET of resource selection scheme 1 are set to 1under the same assumption as in FIG. 38.

Resource selection scheme 2: An SR-UE may be configured to select i) anarbitrary D2DSS transmission resource, ii) a resource having lowestdetection energy (i.e., Min(power)), or iii) a resource having energylower than a predefined (signaled) threshold value, since there is noreference resource (e.g., a resource on which a D2DSS is detected) fordetermining a D2DSS transmission resource based on resource selectionscheme 1.

FIG. 40 illustrates a BS and a UE which are applicable to an embodimentof the present invention.

If a wireless communication system includes a relay, communication in abackhaul link is performed between the BS and the relay andcommunication in an access link is performed between the relay and theUE. Accordingly, the BS and UE shown in the drawing may be replaced withthe relay according to situation.

Referring to FIG. 40, a wireless communication system includes a BS 110and a UE 120. The BS 110 includes a processor 112, a memory 114, and aRadio Frequency (RF) unit 116. The processor 112 may be configured so asto implement the procedures and/or methods proposed in the presentinvention. The memory 114 is connected to the processor 112 and storesvarious pieces of information related to operations of the processor112. The RF unit 116 is connected to the processor 112 and transmitsand/or receives RF signals. The UE 120 includes a processor 122, amemory 124, and an RF unit 126. The processor 122 may be configured soas to implement the procedures and/or methods proposed in the presentinvention. The memory 124 is connected to the processor 122 and storesvarious pieces of information related to operations of the processor122. The RF unit 126 is connected to the processor 122 and transmitsand/or receives RF signals. The BS 110 and/or the UE 120 may have asingle antenna or multiple antennas.

The embodiments of the present invention described hereinabove arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless mentionedotherwise. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in the embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obviousthat claims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentinvention or included as a new claim by subsequent amendment after theapplication is filed.

In this document, a specific operation described as performed by the BSmay be performed by an upper node of the BS. Namely, it is apparentthat, in a network comprised of a plurality of network nodes including aBS, various operations performed for communication with a UE may beperformed by the BS, or network nodes other than the BS. The term BS maybe replaced with the terms fixed station, Node B, eNode B (eNB), accesspoint, etc.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be achieved by one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor.

The memory unit is located at the interior or exterior of the processorand may transmit and receive data to and from the processor via variousknown means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the SR signal transmission method for D2D communication in awireless communication system and the apparatus therefor have beendescribed focusing on examples applied to a 3GPP LTE system, the presentinvention is applicable to various wireless communication systems inaddition to the 3GPP LTE system.

What is claimed is:
 1. A method for receiving a synchronizationreference signal for device-to-device (D2D) communication by a firstuser equipment (UE) in a wireless communication system, the methodcomprising: receiving a plurality of synchronization reference signalsincluding a first synchronization reference signal and a second ofsynchronization reference signal in a D2D synchronization referencesignal transmission period, wherein the first synchronization referencesignal is transmitted by a cluster head for D2D communication and thesecond synchronization reference signal is transmitted by a second UEbelonging to a cluster for D2D communication.
 2. The method according toclaim 1, wherein a transmission period of the second synchronizationreference signal is configured to be different from a transmissionperiod of the first synchronization reference signal.
 3. The methodaccording to claim 1, wherein a transmission resource index of thesecond synchronization reference signal is configured to be differentfrom a transmission resource index of the first synchronizationreference signal.
 4. The method according to claim 1, wherein the firstsynchronization reference signal is repeatedly transmitted to beequalized with a boundary of a subframe when a random backoff end timeof the cluster head is not equal to the boundary of the subframe.
 5. Themethod according to claim 1, wherein resource allocation information forthe first synchronization reference signal and the secondsynchronization reference signal is transmitted over a physicaldevice-to-device synchronization channel (PD2DSCH).
 6. A first userequipment (UE) for receiving a synchronization reference signal fordevice-to-device (D2D) communication in a wireless communication system,the first UE comprising: a radio frequency unit; and a processor,wherein the processor is configured to receive a plurality ofsynchronization reference signals including a first synchronizationreference signal and a second of synchronization reference signal in aD2D synchronization reference signal transmission period, and whereinthe first synchronization reference signal is transmitted by a clusterhead for D2D communication and the second synchronization referencesignal is transmitted by a second UE belonging to a cluster for D2Dcommunication.
 7. The first UE according to claim 6, wherein atransmission period of the second synchronization reference signal isconfigured to be different from a transmission period of the firstsynchronization reference signal.
 8. The first UE according to claim 6,wherein a transmission resource index of the second synchronizationreference signal is configured to be different from a transmissionresource index of the first synchronization reference signal.
 9. Thefirst UE according to claim 6, wherein the first synchronizationreference signal is repeatedly transmitted to be equalized with aboundary of a subframe when a random backoff end time of the clusterhead is not equal to the boundary of the subframe.
 10. The first UEaccording to claim 6, wherein resource allocation information for thefirst synchronization reference signal and the second synchronizationreference signal is transmitted over a physical device-to-devicesynchronization channel (PD2DSCH).